Subwoofer Choice

As much as I might respect Rel, I think you should take a hard look at a pair of SVS SB1000 Pro sealed subs.  They’ll do 20Hz (-3dB) as opposed to the Rels only going to 27Hz (-6dB), and the SB1000s, like the Rels, also accept speaker-level inputs.  Plus, the SVS subs come with integration software and a 45-day, risk-free trial with shipping included both ways so you can see if they work for you.  Now, in addition to all this, here’s the kicker — you can get a pair of SB1000 Pro subs for $100 less than ONE T/9x.  I’ll take two good subs over one 7 days a week and twice on Sunday.  Hey, it’s your money so spend it as you wish, but to me the choice couldn’t be clearer, especially since you can try the SVS subs at home totally risk free.  Best of luck. 
https://www.svsound.com/products/sb-1000-pro-subwoofer?wiz_medium=cpc&wiz_source=google&wiz_campaign=760424595&gclid=EAIaIQobChMIl8aN54LY_QIVCfzICh1bwAqeEAAYASAAEgIJcPD_BwE

2 always two with out question. You will not need that much output for music. My JLs run at about 10-o-clock on the dial with 90db speakers. 

How do you plan to connect them? that is far more important than what sub you choose. 
 

Low passing them under your mains is fine but not ideal. It is complicated to get right and honestly makes things worse imo. If you highpass you can take the system to another level entirely. There are lots of cheap ways to highpass (mini DSP, etc) and more expensive two channel oriented ways (JL-CR-1, wilson sub controller, Bryston, etc). 
 

anyway, I would add SVS to your list. They are a better sub imo. The SB16 ultra is an impressive sub ($2300ish) with lots controls and a bit of built in EQ. They can be controlled from a phone app. I think they are comparable to the fathom line at 1/2 the price. They have cheaper options that sound as good really, you just lose features. I personally run JL which sound good but I can’t recommend them due to reliability issues. 
 

For me I would step down to the Sopra 2 and add a JL CR-1 (active analog controller) and two better subs of your choice and never look back. Passing to the subs around 60-80hz makes for a very impressive system. This will allow the bass to perfectly match the room as it is 100% adjustable. Looking at the Sopra’s measurements I would highpass them right around 65hz as a starting point. Try 10hz lower and 10hz higher and see what direction you like. 
 

@james633 

Keep in mind that adding an active crossover is just another piece in the signal chain which if you are ok with that. I am not discourage anyone from not using a crossover and honestly I think it is great with the flexibility if someone knows how to dial it right.

I think that many of us have limited experience with directly comparing subs. I’ll count myself as one of this crowd, but I went through a similar selection process last year. Up front I decided that if I was going to go line level then I would want a very high level crossover like the Wilson’s, but that killed my budget. Second, I decided that I wanted a pair of subwoofers to avoid room nodes and make setup/integration easier, so I settled on REL and high level inputs. I looked at their smaller subs, but found that as I went to their more expensive subs their drivers/amplifiers became faster and the 3dB down point dropped a bit. I reasoned that faster was important when integrating with my speakers.