I'm guessing the headphones you have offer a better bass response than the HS5's do.  I think the HS5's do an OK job but I haven't hooked them up lately.  Maybe check out the master EQ and roll off some highs and see if you can "compensate for the room" (I mean monitors).   Master EQ isn't a global setting (at least I don't think it is).  This may change with newer firmware.  So assuming it's not then this is something you'd have to modify on a per-Performance basis and [STORE] your own compensated Performances.

Getting a good match is always a tricky thing.  "The wrong" (which is a subjective thing) monitors can really make or break the sound.  

If EQ is going to help then I guess the decision to make is if getting an EQ unit (hardware) makes as much sense as getting studio monitors that deliver more bass response (and/or adding a subwoofer).   The Montage Classic demo units I saw were always setup with an HS8S (subwoofer).

Earlier in the thread there's discussions about the velocity curves (which are global system settings).  Some do not like the sound of the higher velocity samples and have reported taming the response by using velocity curves or even editing the Performance where there is more control at the element level to prevent from higher velocities from triggering those more aggressive samples.  I don't think it's necessarily what's going on with your description - but just a nod to this possibility.

Thank you very much, Toby, for the response and points to follow. I've checked them and they don't change.

Thanks so much, Jason, for your reply.

Playing with the EQ and curve speed somewhat mitigates the metallic effect of acoustic pianos.
The improved bass reading provided by the headphones is a good answer and helps me understand the difference.

Considering everything I've read from other users and specialists, plus other technical questions I've asked:

My system setup isn't ideal for hearing the Montage M6 acoustic pianos well.
Montage is connected to a Zoom Livetrack-L12 mixer and from there to Yamaha HS5 monitors.
The Zoom creates a bottleneck at certain frequencies, altering and coloring the Montage M6's sound in a way that doesn't enhance its naturalness. Furthermore, the HS5s can't deliver the bass that the Montage M provides and emphasize the mids and highs.

I'm no expert; I'm new to Montage, and I'm struggling to understand all the parameters that affect the Main Out. Indeed, I'll have to adjust/edit/modify/remove effects or add them when I learn, and of course it will affect the sound for the better.

So is this a problem with the Montage M? I bought it thinking that when I plug it in at home it would sound like the promo videos or something similar.

Perhaps Yamaha is missing a warning: "Samples taken with such equipment, subsequently edited and equalized with professional production...the sound in your home may not be the same." "Recommended equipment with these minimums." "If you notice a metallic sound, it could be due to this and that, and the solution largely involves equalization and such."

Would this be an acknowledgment of a factory defect?

Not all Yamaha customers can be experts, and this makes me very disappointed with the product they deliver.
I'll buy other monitors on par with the HS8s and remove the Zoom Mixer to install a quality audio card.
Best regards, everyone.

No - it isn't an M problem. The M models have a very good audio interface.

More likely it is an environmental problem: speakers, speaker placement, room dynamics.

When you use headphones they take all of those ancillary things out of the picture.

So is this a problem with the Montage M? I bought it thinking that when I plug it in at home it would sound like the promo videos or something similar.

Except there is usually little, or no, info about those ancillary things that were used for the promo.

One of the more common questions users ask is "why doesn't my playing of a preset performance sound like the AUDITION?"

An important thing to remember/consider is that a synth uses programming for EVERYTHING it does. And the use of scenes, arps, motion sequences, knobs, sliders, controllers can all affect that programming. 

How hard you hit a key can affect the velocity programming.

Rather than consider presets as 'ready-to-go' backup/rhythm sections consider them as demos/examples of what the instrument is capable of doing when you stretch it to its limits.

Not all Yamaha customers can be experts, and this makes me very disappointed with the product they deliver.

A synth like the M's can, and do, have a fairly steep learning curve. Your frustration is understandable and we all go through it. And when a new update is released (OS 3.0 due at the end of June) we go through it again to figure out just what is new, how to use it, and how it works. 

I'll buy other monitors on par with the HS8s and remove the Zoom Mixer to install a quality audio card.

That should eliminate any 'bass' issues. The standard HS5 and HS8 dual speakers really don't provide the bass needed for a full sound.

But that also illustrates one of the problems in configuring things to get the sound you want. The sound you need for your home isn't going to be the sound you need in a bar, auditorium or other performance venue.

One size does NOT 'fit all'. That is where judicious use of good headphones can play an important role. When you use headphones you get a much better idea of what things will sound like when you change REVERB, DELAYS, ECHO, PAN and similar things.

You can test configurations for different size venues using headphones that you can't really do with just a pair of speakers.

If you do get some bass speakers post back and let us know if that addressed your issue.

Thanks Toby, I'll take note of everything.