You say TRS - then you say guitar cables. They're not the same. Guitar are only TS - no "R"ing. As an analogy - the 1/4" TRS connectors look like 1/4" stereo (3 conductor) and 1/4" TS cables look like 1/4" mono (2 conductor). Guitars "all" use the 2-conductor type. At least when you say "guitar cable" in any studio - you'll get a 2-conductor 1/4" cable.

Still, it doesn't matter as far as breaking things which type you use. TS or TRS. TRS end-to-end will be more immune to noise and generally can see longer runs. The shorter the run - the less you worry about noise either from outside aggressors or from loss/attenuation in the cable itself.

Quality can go into the materials/workmanship. Mostly we're talking about longevity of the cable. But if you get bottom-barrel - the quality or gauge or impedance or other aspects of the cable may impair the signal path. I don't buy into that if you pay a million dollars for a cable - it's going to sound much better than a standard "built to spec" cable with middle-of-the-road materials.

Go ahead and use guitar cables if you want - but don't call those TRS because they're missing a conductor for that designation. Hopefully the cable isn't super long. If you have guitar patch cable (short) and that works - meaning your equipment is spaced far enough apart - then use that. Having extra cable bunched up has the potential for degrading your sound. However, since it doesn't hurt to hook it up even if it's a mile long cable - just do it and listen. Decide if the sound is horrible, acceptable, or outstanding. Adjust if needed. I would personally start with what I have even if it wasn't ideal. I've got some radio-shack grey coil guitar cables (around 6' length uncoiled) that are horrible - but they do the trick in some applications.

-The UR28M will be the only thing connected to the computer via USB. Everything else will be connected to the UR28M.

At some point you will want to be able to deliver MIDI clock to your MONTAGE. Because so much of the Motion Control Engine can be tempo based: Motion Sequences, LFOs, Effects, Arpeggios, etc. So you will need to connect the MONTAGE via USB to an available port on your computer, if only for MIDI communication.

-The montage will be connected to the UR28M via 1/4" TRS cables. I will need TWO 1/4" TRS cables from the montage to the UR28M (into 3/4 line spot of the UR28M).
-All speakers will be connected to the UR28M via 1/4" TRS cables or 1/4" TRS to MALE XLR converter cables. Headphones can be connected to the UR28M only.

Headphones can be connected to either device. What you will be able to hear in each will differ. What is in the Phones when connected to the UR28M will be all the signals coming into the UR28M (from live instruments/microphones and the computer audio returns). The only thing ever in the MONTAGE Phones, in this configuration, will be the MONTAGE.

-Since the UR28M will be my Audio Interface, the montage will not also be able to act as an Audio Interface. Because of this, I can't keep my monitors connected to my Montage.

Your will need to connect your Monitor speakers to the one device that you want to act as your principal audio interface... all analog devices must connect to it... that is the optimum setup. It allows you to hear your MONTAGE even when not recording it... You'll need that.

-I can't have both the montage and UR28M connected to the computer and be active through my computer at the same time. The only way is to route the montage through the UR28M.

That is not a completely true statement. It depends on your computer type. If you have a Macintosh computer you can connect both devices and have 32 inputs to your DAW from the MONTAGE plus the 6 inputs from your UR28M. You would build what is called an AGGREGATE AUDIO DEVICE with 38 Inputs and a stereo Out. That Stereo OUT would need to go to the one device that you choose to be your principal audio interface - the one that connects to your monitor speakers...

For monitoring purposes you still would need to connect analog out from the MONTAGE to two channels of the UR28M (simply so you could monitor the MONTAGE through your speakers). In this situation - both devices can send audio (digitally) by their own USB cables TO the computer... the Macintosh simply gathers all the audio coming in and creates ONE BIG AUDIO DEVICES that represents both of your units.

But that still doesn't change the routing of the analog signal. You could record the Montage (digitally) but would be unable to monitor it direct, unless you connect to the principal audio device.

-Connecting headphones to montage will no longer pick up signal. I'll need to connect all headphones to the UR28M.

Again, the Phones of the MONTAGE will always contain the exact same signal that is available at the MONTAGE Main L/R Outputs. It simply will NOT contain any of the data returning from the DAW to your audio interface!

Just think in terms of ANALOG and DIGITAL signals. RULES are simple: you cannot hear Digital signal... it travels through a USB cable TO and back FROM the audio interface. The analog signal can ONLY be heard when sent to a pair of transducers (speakers) that take electrons and put air molecules in motion.

Do you have a Mac?
Remember even though you can deliver digital signal from both, TO the computer DAW, (38-in/2-out) in order to monitor your MONTAGE direct and data already recorded (Like when OVERDUBBING), you will still need to create an analog signal path to the speakers... MONTAGE L/R Out >>> In to 2 channels of the UR28M.

From my own experience I strongly advise that you buy the balanced TRS leads as I initially used guitar leads which I had laying around and these caused quite a loud hum through the monitor speakers which totally disappeared when I replaced them with TRS leads. You do not really need to spend a fortune on the leads, mine were about £8 for 2m length.

I used TRS here in my studio and unbalanced at a gig since sound guys always want to DI a keyboard. XLR outs would make things easier since 1/4" TRS is a bit of an "odd man out" as far as what they generally deal with. It would be great to see an Neutrik NCJ6FI-S type jack for all I/O including A/D input (w/phantom). There is a price/real-estate hit so I can understand why we're moving slowly in that direction (unbalanced 1/4" before, now balanced 1/4"). Maybe eventually the keyboard will receive a standard XLR and those backline guys can stop placing DI boxes between.

Point being - I agree to use TRS if possible. But I would still try what you have and see if it works first.

Thanks everyone...this was very helpful.

Hadn't realized the finer points of the TRS/TR cables. I guess that was my question...if those 1/4" cables are universal. I will go with the TRS cables for Synth purposes.

Bad Mister, I sort of forgot that I'll need to directly connect the montage to my computer to tempo sync...this is really important so I'll need to reconsider this aspect.

What you describe with the Aggregate Audio Device would be an absolutely perfect compromise...I have a PC, so not sure the dual digital USB routed directly to the PC will work though?

But I would love if I could route the Montage and UR28M to my PC for digital signal separately, and just have the Monitors routed from my UR28M for monitoring purposes only.

I may just try a few different setups and see what works best.