You are using the words "sound quality" which may not be actually what you mean. Certainly, any three controllers with different actions will cause you to feel differently about the response of the tone generator. Without dealing with brand names, know this: a keyboard designed as a true MIDI controller will probably have as a priority working well with external tone engines... While a keyboard designed with its own built-in tone engine will, quite naturally, be optimized to make its own tone engine respond well, as a priority.

Sorry, I don't know the three products you mention, but what you describe does not surprise me at all. First thing you want to look for is the adjustment to Velocity Curve. This will customize the controller's velocity output in response to your effort as input. Typical settings might include "soft", "normal", "hard", "wide", "medium", "fixed" or other such terms...

In general, a "soft" velocity curve is one where you do not need a lot of effort to make the response of the sound engine loud. You'll never break a finger nail. While a "hard" curve is one where it takes a lot of effort to get a loud response, and is designed for the heavy-handed among us.

Try each of the curves... Most of them will be fairly uncomfortable and will feel like wearing some one else's slippers, however, one of them will probably feel like it is a perfect (or near perfect) fit. It is not the sound quality of the tone generator that is changing - it is your feeling about how the tone engine is responding. Have a band mate listen as you move from controller to controller, they will most likely tell you the Rack XS sounds the same to them. What you are experiencing is the "feel" of effort in and response out.

Start with the keyboard controller. Some will have this type of setting. Some may not. But just because a keyboard has a MIDI jack on it does not mean it is really a controller in the true sense of that word. A true controller will allow you to adjust this Velocity Curve, it will most likely have a Mod Wheel, and a way to bend Pitch, and assignable pedal jacks, etc. just having a MIDI jack does not a true controller make.

I cannot help with where to go to find these (contact the manufacturer in question, or look it up in the Manual) it is one of the most overlooked features of modern keyboards... The Velocity Curve. When I hear people say they don't like the action of a particular keyboard, if they have not explored the Velocity Curves they have yet to really intelligently try out that keyboard. The so called "normal" curve is not a statement about anything good or bad... As I always say, it's OK not to be normal. There is nothing wrong with being a heavy-handed player, as long as you know you are... There is nothing wrong with having a very light touch, as long as you know you do... Simply adjust your keyboard controller to fit your playing style.

I highly recommend that you try each Velocity Curve setting for at least 15 minutes to a half an hour. Play all types of things, from scales to actual songs, take your time. Do not be in a hurry to discard any of them. If you doubt that it is the velocity curve, and you still think it is the sound quality that is changing... Recall the Voice "16 + 8 + 5&1/3" this B3 sound has a fixed velocity and you will notice the "sound quality" will be exactly the same from all three keyboards. Have some one else play as you listen...

Most professional Yamaha keyboards feature this Velocity Curve option but it is not exclusive to just Yamaha... The keyboard is the place to make the quickest and easiest change to your playing experience. Most Yamaha tone generators, including the Motif-Rack XS, allow you to adjust the response on a per program basis. So that you can customize each sound you select to the incoming velocity. Just as the organ Voices do not respond to velocity, each sound can be individually customized (to a very dramatic degree) as to how it responds to your controller. This is rather in depth editing, (with the Rack XS you'll want to use the free Yamaha Motif-Rack XS Editor), but in all likelihood, this will not be necessary to fix what you have described... But it is an option available to you.

Thanks a lot for your answer, I´ll check on the velocity curve of the keyboards to see if I get any improvement.