Mine is the 21SE. There is a new one on here for $350 and an 11 for $90. The 11 would be far better than the amps that are built into most amps or receivers and would be a good choice, the 21 is better and has other functions as I mentioned.
Try and pick up a used X-Can V2 by Musical Fidelity. Should be reasonable here on A'gon, I drive my HD 600's with one, and it's very listenable. There are newer varieties of them, and, they are going to sound better, but then they will cost more, too.