What would you use

...to distinguish the brands of a piano or grand-piano playing.
mentioning interconnects or speaker cables isn't neccessary.

as far as i realize that even electiral keyboards have it's own sonical signatures.
Well, first of all, you have to know what the piano sounds like in the first place. Anyone who has ever heard a Steinway grand knows the unmistakable "Steinway Crash" in the midrange sound when heavy impact chords are being played. A nine foot Bosendorfer has a signature of unmistakable bass power with less noticeable hammer impact. No midrange crash. A pianist or piano aficionado can tell instantly whether a grand piano or an upright or spinet is being played, by the resonance qualities. I could tell you with electric keyboards whether it is a Roland, or a Yamaha, or a Kurzweil, or with synths, an Oberheim, or a Prophet, or an Arp, etc. It is simply a matter of experience. Obviously, the better the recording and playback system, the easier it is to tell what is being played. But if you don't know the sound of the instrument, then you'll never tell what it is by the recording. Most guitar players and even keyboard players like myself can tell when a Les Paul, or a Telecaster, or a Strat, or an ES 335 is being played. On acoustics, a Martin sounds different than a National. Etc,etc....
Twl above speaks for me, too. To add a technical note, it helps if the system correctly reproduces harmonics, timbres, has pitch... Of course, a better recording helps!
...that's what i've started to realize.
for ex: i could now distinguish Yamaha from any other keyboard; I could now distinguish ES125 from any other guitar and certainly upright piano from grand.
Electric instruments can be very tricky. It may sound like a Wurlitzer electric piano, but one would be hard pressed to say whethers it's a real Wurlitzer or is it a Korg Trinity (or any other number of synthesizers) running a Wurlitzer patch? Solid body electric guitars have very little sound of their own. The pick up design and the amplifiers used are a greater determinate of the final sound than the guitar itself. Couple this with the choice of microphones and placement and a Telecaster can easily be made to sound like a Les Paul (Led Zep I & II).
Onhwy61, I would agree it can be tricky with electronic instruments. However, unless the keyboard man is a real whiz with his sampling and editing, you can still tell. With analog synths, the patches can be similar sounding but the sonic character of each will show through. If the programmer is really good, he can simulate other synths well, but nobody can make anything else do patch 33 on a Prophet 5 and make it sound the same. Oberheims, Moogs, Arps, and others have certain sounds all their own too. Of course, none of this matters if the music is enjoyable to listen to. It is just a challenge sometimes to be able to identify the instrument as kind of a fun thing to do.
Twl, if you're looking for a challenge, try picking out the number (and of course type) of pianos used in any of those 60s Phil Spector masterpieces. Personally, I can't tell, but I read that he typically used 3 or 4 at a time. I think he also used a similar number of acoustic guitars. Talk about a wall of sound.
Onhwy61, that would be a challenge. I seriously doubt I could even come close. But, in small scale music, I can do it alot of the time. I guess it comes from playing in a band, and always paying attention to the sound of everybody's instrument. Since I was the "techie", I got to do the recording and mixdowns, in addition to playing keys. When you do that, you get pretty familiar with the sound of the different instruments and effects units. Alot of times, I used to suggest the effects patches and stuff for the guitar player. Of course, he always told me that I was a keyboard player, and what do I know? You know how those guitar players are.
Twl, nothing was more irritating than having the piano player tell me to tune my guitar. Enjoy!
Marakanetz, if you're asking about which speakers would one use to distinguish between different pianos, I once gave a demo to a piano instructor and she could readily tell me what kind of piano was playing. We were listening to a pair of Sound Labs. Unfortunately, the piano instructor was a B&B guest rather than a customer, having taken the customary vow of poverty.

Another time I bought a disc of a local pianist (Seth Kaufman), then later heard him live. Now, I'm no piano expert, but it sounded to me like the same piano, so I after the performance I asked him if the recording had been made on the same piano. He said yes, and added that he'd had his piano shipped from New Orleans to Los Angeles for the recording session.

I'm not saying the Sound Labs are the only speaker that can differentiate between different pianos, but they are among those that can.

Another speaker that I think would have a good shot at pulling it off would be the Quad 57, especially stacked pairs.