I've never been too keen on subwoofers for music, but now after many years I have found one.
I think the only reason it works though is that a) it's designed to go with the speakers, b) it's expensive, and c) I'm prepared to position it out in the room rather than dump it in the corner.
It's easy to get the subwoofer wrong and make things sound worse.
Given your system and budget, and that the 685 should be sold easily for a good price, I'd consider just changing the main speakers.