Research for subwoofer

+2 for Rhythmic.   I have two and they are a lovely addition to both my Totem Element V2 and my Xstatic.  They are easily tuned and adjusted to pair well with any speaker.

Easy, I’d either go with Rel or Rel. ;-) 

I recently got a pair of Elac Varro RS700. They are well engineered, fast and musical subs that go deep. I believe front firing sealed box subs are more accurate.

FWIW: Even though your Luxman has pre out, it is generally recommend to use high level outputs for better integration of subs with speakers. REL, Elac and others offer this option of piggybacking wires off speaker terminals to provide signal to the sub, which the sub then amplifies.

Whatever you choose you can look forward to a much more satisfying hifi experience!

@gkelly 

+1 

Gonna have to agree that that’s a pretty significant discontinuity between quality of amp vs. speakers.  There are no absolute rules, but it’s probably more common to spend more on speakers than on an amp.  You can get pretty awesome amps for under $5k, but really hard to do same with speakers.  I’d address that issue before investing in subs.

add my 2 cents

pair of rel t9i or t9x - use the high level connection... a pair of them would be ideal in your larger open plan room