Research for subwoofer

are you asking about what brand, how many subs, placement and all that? 

Depends greatly on the budget. I like the SVS-KEF-REL trio comparisons, there are a few of them on youtube. I am sure Velodyne and Rythmik are good too. 

I wasn't too happy with my home audition of ML, SVS, Klipsch and Polk Audio subs. Ended up with the REL and never thought of subs again.

What’s your budget, and are you looking for new or used?

Room size, and treatment, if any matter.  As always, buying and integrating a subwoofer is some real work.  Some get lucky and some really have to adjust.  Also, check the AM Acoustics Room mode simulator.  it will help you place your speakers,sub and listening location optimally

Brand yes type yes (powered or not?)

New or used - Either is ok.  I have had good luck with used from Audiogon members

Thanks for the AM Acoustics recommendation

Thanks for REL and SVS recommendation