A used REL strata 3 is quite good for that price. You might even get one of their larger models. RELs are very subtle ... to the point where I often can't tell that mine is doing anything until I turn it off and the music loses all depth. Integration is that good. Subtlety and integration are the keys with classical music.
You really need to tell us your main speakers and your room size, and whether you have a preamp output to which to connect the subwoofer in order for others to make other suggestions.
You really need to tell us your main speakers and your room size, and whether you have a preamp output to which to connect the subwoofer in order for others to make other suggestions.