Which subwoofer should I get?

Taking it one step farther, Wilson and JL both use their own active crossovers with their subs. 
 

link to wilson’s. 
 

https://www.wilsonaudio.com/products/activ/activxo

Westminster Labs Shockwave, currently in 12" and 15" configurations, pending or already released new 10".  Takes the monitor speakers and even full range standing speakers to a new level of fullness and openness (bass effects highs as well).  Brilliant design and execution but not inexpensive.

mwsl

Hi Matt. Just to clarify what audiorusty advised you. The Rel T5x you were originally recommended (in your first post l believe) would not cause concern with your A5’s. The T5 range do not have the passive radiator, just a single down firing driver. Move up the T range and they do have the passive radiator.

My only concern would be if the T5x could be too small for your room but if you are thinking of a stereo pair then that is a different situation. Try one and if needed buy a second….they are not that expensive.

In one my of my replies to your post l stated that l started with a Rel Quake the basic 100 watt budget end of their range. It was very impressive with Hi-Fi and Cinema so l bought a second. They integrated so easily and once set up there was no need to endlessly fiddle. I kept them for over ten years. The T5x is about the same size as the Quake but from memory has a 150 watt amp. T5i’s are a cheaper option on the 2nd hand market too and maybe not a lot difference to the T5x.

l believe you may have moved up the Rel range in your considerations now but thought this may still be of help. The Rels are well made and the cabinet finish is exemplary.

@ Audiorusty - thanks for validating James633. It struck me as good advice as well

@ mylogic - truly appreciate your ongoing clarifications. I was originally thinking about the REL Carbon special. 

To all: I am now convinced that a pair of subs is the direction I will be going. My overall budget needs to be limited to $4k, which gives me lots of options. I’m still leaning towards Rhythmik (wish they had app control, but I’m a little less hung up on this as being a critical feature…..more of a nice-to-have rather than need-to-have). The Arendel 1723 S1’s looks like another very interesting option, given their app control and better aesthetics. 

Once again, very appreciative of the advice this group can offer. Will keep you apprised later this month post-construction (there’s going to be holes cut in our drywall to install a new window; don’t want to introduce new equipment until the construction dust settles).

@ fleschler - FYI the Shockwave subs are a Von Schweikert product. Definitely over my price point. FWIW Westminster Lab makes amplification and pre-amps.