Welcome to the forum, I have a 2100 classic and I have the usp1, what sounded terrible about the 2100? If it was adding or taking away anything from the sound, there is something wrong, were you using the bass boost or tone control? For the most part the preamps and amps all sound the same, I have seen speakers reproduce material in different ways but amps and processors {unless you are eq'ing} should all sound about the same, as far as my ears can tell anyway...
I like my 2100 and I like my usp1, but if I had to choose between the p5 and the usp1 {don't forget the p5 has a built in dac}, I would rather have the usp1 for half the price, actually rite now you can buy a usp1, an xda2, and a umc200 for around what a p5 costs, so that would be the route I would take, obviously...