Subwoofer Choice

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.

I'm a big fan of REL products. If it's within your price point I'd recommend purchasing a pair of the S/510s. If that's not doable purchase one S/510 now and the second one when you can afford it. 

Highend64 

yes agreed, adding a crossover could possibly be heard. I can only speak of the JL and my system (Mcintosh Revel) I can’t hear a change at all. If you have a very highend system maybe it would be different. The down side is good analog crossovers are expensive. Digital one are fine in theory but I get concerned about the quality of the output stage of these cheap digital units. I mean every time I change interconnects or try some “giant killer” wire it degrades my sound….

I was unable to get smooth bass (measured and heard) running the sub off the preamp crossing under the mains. But using a crossover got my room to +/- 6db in the bass in an afternoon. My bass is pretty flat now with a 6db hole from 64-71hz. I EQed out that dip but thought the EQ hurt the sound quality more than the dip so I just left it. I can’t hear that dip but it shows up on measurements.