The 5503 is a powerful device indeed, the biggest problem is the size of the wave tables. The smallest buffer entry is 256 bytes and the biggest is 32KByte. If I wanted to do a piano I would sample a piano for each octave (maybe three to be a bit skimpy), that would be 96Kbyte with the highest quality, and if we half the sample rate 48KByte and if we half it again 24 KByte. Still quite a lot for a 64K/128 KByte machine. 256 bytes can be awesome for gun shot sounds etc. With moderate sample rates and wave table entries you could probable make some really nice music as well.
Still it would be really fun to play with such a device, i found a Canadian distributor that has the devices in stock..... Anyone.......