Another vote for the Vandersteen 2wq sub. I owned one for a couple of years, and bought another. The crossover setup makes this the most seamless sub system I have heard. I have used them with Quad ESL-63 and Snell Type A speakers. These subs are wonderful, and they are subtle. I listen to a good bit of classical, largely piano. Chopin, mostly. These subs do not draw attention to themselves but create the wonderful "feel" of the room acoustic ... that intangible (it's actually nearly tangible!) foundation.
One caveat, and I would love to be educated further on this by the Vandy dealer that posted... once I tried these subs with speakers that had sensitivity of about 91 dB and I could not dial the sub output high enough to keep up with the mains. Therefore, unless I am missing something these subs are the best you can find for mains that are 89dB or lower sensitivity and completely unusable for 91db sensitivity and higher.
Regarding crossover, I bought the MH-5 outboard passive used in the Vandy 5a and they are transparent and amazing. I bought one used for $600 I think. So if you can find a 2wq used for $600 and an MH-5 for $500 you are close to budget.