I'm not sure any of this is relevant in a Yamaha Synth forum, but since we are here...

The sky is the limit in terms of what you can pay, and for what is considered by you to be "adding value".

You must consider your audio reproduction equipment as a "Single System". If any component of that system is of lesser quality, then the whole system is limited to the quality of that "lesser" component. A classic example is "old" or "$2 Bargain" cables. Your $20000 system only sounds as good as that $2 Cable.

What do you do? Buy a $800 Silver Stranded, Gold Plated Connector, Mylar and Kevlar insulated cable? You can if you want to, I.e. if the perceived benefit is worth it to you.

Back to speakers. All speakers "add colour". Studio speakers strive to be FRFR... Full Range Flat Response... which in effect means they add no distortion (change in shape) of the Input waveform (Audio Signal). However they should be labelled "As close as we can get to FRFR with available technology, materials and reasonable cost".

Besides, Human ears don't like FRFR, they prefer "coloured" or "sweetened" speakers. And everyone has a different favourite colour.

The FRFR approach, is really used to provide a consistent "base", "datum" or "reference" line, allowing producers who are building mixes daily, to build off a level playing surface. They don't build mixes to sound "the best" through FRFR Speakers. They learn to build mixes that sound "odd" on FRFR, knowing they will sound "good" through typical "home hifi" or "sweetened" speakers.

If I were OP, I would research and test whole "Systems" to locate a preference, rather than keep throwing money at disparate "components".

Take your YC down to a good Audio/DJ/Studio store, and request for trials on proven "systems".

Your input to the "system" will be Left and Right XLR cables. Presume that's as much as you know for the moment.

From there you could be looking at Rack Amps, Mixers, Monitors, PA, Headphones... who knows. It's a considerable investment, so worth the research.

Some more food for thought.

On GearSpace forum, there are some members with $100k of studio gear who slate the digital VA, ASM Hydrasynth, as sounding cold and harsh.

I only have $30 cables, a $400 5" Speaker pair, and a pair of $120 Sony Headphones. I use direct output Jacks on the Hydrasynth.

I don't hear cold and harsh, I hear warm lows, buoyant mids and a sizzley top end. It sounds good to me and in all honesty, that might be because my Audio reproduction "system" is cr@p... at least by their standards.

Another perspective. For my Guitar I have a Vox UK Factory custom built AC30, with UK Celestion G12 "Blue" Alnico speakers. The amp and speakers are known to be "flawed" in terms relative to correct and efficient Amp and Speaker design. To say these amps are "coloured" would be a massive understatement. But they sound fantastic, very "musical", they are arguably an "instrument" in their own right.... and they are not cheap.

I also have the Hydrasynth Deluxe but I spend most of my time on the playable YC. I like the HS for textures and features but mostly just play traditional instruments if pressed for time. Yes generally a producer doesn't want the drivers to become an instrument for obvious reasons. Yes adding driver instruments is also fantastic. Once I stacked about 6 powered speaker systems driven via a fiber optics hub justs to hear what all of them brought to the table in unison. Reproduction into air and space brings in room ambiance and reflections and endless variables however headphone remove all of that so it makes sense to reduce it if you want to instead of 100K on an entire acoustically treated studio. My experiment was to avoid the 100K studio, system etc. and see what top shelf phones would do. It has improved the YC enjoyment by double to hear nuance and frequency and range and clarity I've never heard before. It is like a 100K system shrunk down to a 3K personalized system on your head that I can still tweak it using the YC equalizer. I am using the R.Neve amp which provides more power not that I have to use it for the Utopia 80ohms vs the Beyerdynamic 250ohms. If you read about best phones you'll eventually hear about which phones go best with which DACs/amps and on and on. It does never end but fun to experiment with. IF Yamaha could include $2K phones with every keyboard people could hear how good the board actually is but what happens is we spend $3k on the board and attach a $100 pair of speakers or phones and then wonder why it doesn't sound great. Going out on a limb here but if your reproduction system doesn't cost at least as much as your keyboard you are not hearing what it can actually do. The get what you pay for principle in action. I'd at least conclude that the Utopia handles all the YC or other boards have to offer in quality of sound. IF distortion and dropped frequencies or muddiness come into the mix then you won't enjoy it fully. Like you said if you have an affordable system in a room then all sorts of other stuff can happen mostly not good and more money is required to fix it. Unless you like that sound and it's all subjective. The best option for elimination of most negatives are top shelf headphones at least as a benchmark or ground zero comparisons. Ignorance or absence is also bliss so if you haven't heard it the best then you don't really know what you are missing. I won't ever play keyboards on cheap phones again just because I've tasted the best. True there are lesser expensive pro phones that add flavor one was described as seductive for a lot less money. That's fine too and also enjoyable.

What you are doing is fine, and it sounds like you know what you are doing also. I was just preaching a little caution, maybe to unsuspecting others, following this thread. Chasing Audio products can get very expensive, very quickly... sometimes with no discernible benefit other than kudos.

High end audio in the keyboard world for me would be a case of pearls before swine. I am just not good enough on keys to justify the cost. All I really need is to be able to hear what I play at reasonable quality (no hiss, pops, crackles etc). I tend to be more focused on my selection of notes (I.e. melody, harmony ... the "music" etc) than the detail in the sound. Sometimes I play my electric guitars unplugged (in a quiet room), because it helps me focus on the composition, rather being distracted by FX and Amp Tone.

Popular Science rated the Neve and the Monolith Liquid Platinum as best solid state and best tube headphone amp. so I'm trying them both. I wanted to compare to get a sense of difference and perhaps warm up the bottom and mids using the tubes though the Focal Utopia headphones. I also 3 stacked my YC73, SKpro and Hydrasynth Deluxe all running into the YC inputs and controlled by YC. Seems to be a really solid rig.

State of the art headphone amplifier, meaning engineering at its ultimate best.

SINAD higher than 121 dB, and nearly 0.6 watts into 300 ohm. High impedance headphones not an issue at all and if you happen to hear any distortion, blame your headphones.

Topping L30 II, around US $149.

I said US $149, yes.

Reviewed on July 26 2022.

https://www.audiosciencereview.com/forum/index.php?threads/topping-l30-ii-review-headphone-amp.36027/

Headphones...?

Dan Clark Expanse. US$ 3999.

Reviewed on September 12, 2022.

I said US$ 3999, yes...

https://www.audiosciencereview.com/forum/index.php?threads/dan-clark-expanse-headphone-review.37332/

Oh, I remember that Audi A4 Avant, Quattro, Tiptronic, 200 HP, 70000 Km (43505 miles) for US$ 10400...

Earphones...? See this ones for US$ 50, oh, please...TOP NOTCH, BEST EVER TESTED...US$ 50 !!!!

Truthear Crinacle Zero IEM.

Reviewed on September 14, 2022.

https://www.audiosciencereview.com/forum/index.php?threads/truthear-x-crinacle-zero-iem-review.37380/

Yes, very cheap indeed and still yet the reviewer said it clearly...

Go and buy one of these! I don't care if you don't listen to IEMs normally. You want this IEM so you know what correct tonality is, when you listen with other transducers such as headphones and speakers. Think of it as the tuning fork that a musician uses! This IEM needs to be in your arsenal.

Mine arrives soon...

😀 😀 😀 😀

It's kind of sad when the masses google "best X,Y,Z" we can only trust what comes up but like you mention there are other options that are a fraction of the cost. Too bad they don't actually compare everything available and give accurate information. Typically the saying "you get what you pay for is a fact". A $150 widget should never be better than a $1000 widget. The little guy never makes it into the mix. I must have searched a million times for best headphone amps and never saw the Topping L30 II. If they'd have made it look cool or something. I have a $40 pair of Edifier headphones and I'm not sure I can tell much difference in that and my $4,000 Focals. I think our brains are only as good as they can reflect information back passing through our ears etc. We probably all hear differently same as we all see differently. As we get older it only gets worse. I probably can't tell much difference between these amps either but it's a fun hobby. IEMs freak me out with all the hygiene issues and nasty stuff getting on them.