Subwoofer Suggestions

About 5 years ago, I put together a system for my college-bound son.  Started with gifting him my restored Pioneer SX-950 (85wpc) receiver, then purchased B&W 607 bookshelfs and a Rega 1 turntable. Nice little system.

He's been in between job assignments recently, so all his gear is now set up in my listening room.  The 607s sound great, but i think they could use the help of a sub.  In all my years with this hobby i have never used one....so i need some suggestions, especially where/if they are compatible with a vintage receiver, what wattage is appropriate, etc.  Any specific brand/model suggestions will be appreciated. Max budget is $600. 

Thanks!