Best SUB that blends into Music not Home Theater

I'm looking to spend about $400-500 for either a new or used. I have a Krell KAV-300i so there is no sub-out. I guess I will need to hook the sub on the int. amp speaker output.

The speakers I have are Green Mountain Europa. Great mid-range but somewhat weak for more active energy music. I am not looking for thunder, but something that blends in well with classical music and such.

Subs I was looking at:
1) B&W ASW 300
2) KEF PSW 2000
3) Klipsch RSW-10 Reference
4) Infinty Interlude IL-120s
Also Check out the Vandersteen subs. I don't remember which, but one is known to be excellent with music and the other with HT.
Good luck.
Here you go Adire Rava..this sub matched perfectly with the Europas during my listening auditions before I bought them.
The Vandersteen 2Wq sub is one of the most musical, reasonably priced subs in high-end audio, and blends very well with the main speakers (which should be fairly full-range, with flat frequency response down to about 40-45 Hz). For more info, use the following link to the Vandersteen web site:
For your under $500 price range the Hsu or the Adire Rava are you best bets.
Hands down the vandersteen 2W.And if you go so bold as two, you will find out what they recorded down there.$500 used each.NO BRAINER!