The best answer is use a keyboard amp for a keyboard, and a guitar amp for a guitar. If you want a setup that is optimal for keyboard bass notes you want a 15" speaker and an amp that is more of a wideband hi fi design. A bass amp might work better than a guitar amp. If you want to emulate a grand piano well, you want to go a step further to a real high fidelity audio system. But even then it still won't sound exactly like a grand piano. I think that if you want a great grand piano sound you probably want a grand piano. People's are fairly well accustomed to the sound of a real piano in a room and that makes it harder to fool the brain. I do think that with a little bit different speaker the Tode makes a great harmonica amp too. But going beyond guitar and harp and you are in uncharted territory.
My bro Art Khu was here a few weeks ago and he thought the midi KB setup I put together using a Fender Rhodes emulator thru our main system was better than his Nord. But it was playing thru my big system, so that probably influenced his opinion. Interestingly the Rhodes had been sampled both DI and thru a speaker/amp. The DI sample sounded great, but the sample from the speaker sounded broken to me. Obviously whoever put out the software thought the speaker sample had merit. So when it comes down to it, each player has to decide what sound they want.
As for Hammonds, I hear everyone about the hassles of moving one around. The samples I tried thru the Midi keyboard were very poor compared to my modded Model E with a Leslie 31H. Presumably there are better emulators, or I am just a hammond tone snob. No way I was moving half a ton of organ gear to our upstairs listening room. So we stuck with the Rhodes sound for the evening.
