Would you choose 1 ou 2 subwoofers for a stereo set?

Another benefit to using subs is the ability to high pass your main speakers, and possibly even mains that go as low as the 802s, thereby relieving the amplifier and the low frequency drivers in your main speakers from trying to push low bass frequencies.

In my case, I was able to switch out larger, full-range bass reflex (i.e., ported) main speakers for better sounding, acoustic suspension (i.e., sealed) speakers that offer much better sounding bass down to about 40Hz, and then roll in the subs from there.

You will only be able to determine whether using a high pass filter sounds better with your 802s by trying it. The result will depend on the HP filter you use, your subs, the ability of your 802s to handle low bass, the type of music you listen to, and the volume you typically listen at.

2 - 12” and my vote goes to REL. For music, REL are so much better and easy to blend with your main speakers than JL’s. 

I had one vented REL and was happy vs no sub. Moved to two 12" sealed Rythmiks and was much happier. Now I have 4x 12" sealed Rythmiks and the improvement has been great again!

If you have total flexibility for positioning subs then 2 might be enough. I don't,  as the system is in my living room, and 4 subs is much better in my case. I also implemented MultiSub Optimizer software and a MiniDSP 4x10HD. When I had 2 subs I simulated in MSO using 2, 3, 4 by moving the 2 subs and taking measurements. 3 was better than 2 and 4 was better than 3. Went with 4 and haven't looked back. End of the subs path for me.

I suggest DSP-based active crossover, time alignment and room correction hardware and software such as Danville Signal dspNexux, miniDSP or DEQX

• Time Alignment in 5uS increments
• Sample Rate Conversation and Jitter Attenuation
• Room Correction and Bass Management
• Active Crossovers
• low pass the subwoofer and high pass the full-range loudspeaker 

I use two 15” sealed Rythmik subs in a very difficult room with great results attributable to DEQX. Use a professional (Larry Owens) to calibrate the DEQX and plan on spending three hours of your time.