Advice Please On Subwoofer Choices For My Secondary System.


Hi all, I'm considering adding a powered subwoofer to my man cave system. It consists of a Cambridge Audio CX A60 Integrated amp, Onkyo CD-7030, Cambridge Audio Network Streamer, Pro-Ject debut  carbon TT with Ortofon Blue cart, Acurus out board phono stage and Wharfedale Diamond 225 bookshelf speakers.  What used subs should I be looking at? I'd
like to keep the cost down to $300-350 tops since I'm in the process of slowly upgrading my main system. Thanks, Mark 
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Thanks for that extensive write up, interesting. I was thinking I could get a good used one for around 300. My man cave is actually quite big, it's a 1200 foot sub basement but I sit about 7 feet away from the speakers at a work table where I listen and work on hobbies. Would a small sub work well in this setting?  Thanks all who have responded.