The volume level of my preamps vary also. Starting with zero sound at 7 on a clock dial, i can rotate my Conrad Johnson preamp to 9, and music is quite loud, too loud most times.
My BHK pre, I am able to turn to 35 to 40 with my more efficient speakers and using xlr outputs, and turn to a higher number with more inefficient speakers with rca cables.
With my parasound preamp, am typically around 62 to 65
with all due respect, Ken Rockwell is not the authority you want to pay too much attention to. He started out as a camera/lens reviewer, but most of his articles/reviews are not hands on but aggregates of what other people have said.
Back to the OP:
Start with one. Most of the recommendations for 2 subs come from HT people. There is a good reason why you want 2 subs to cancel out some room nodes and nulls, but for 2 channel music, the main purpose is to add the extension to your speakers. You already have a pretty decent floor standers. You will most likely be adding sub bass below 40Hz. That means, your cross over frequency will be near the lowest (30hz). I run S/510 which is a smaller brother to S/812 with a pair of bookshelves that roll off at about 48Hz. If you integrate it right, you should not notice the presence of the subwoofer (window shaking, chest thumping). Instead, you will hear much more extended frequency response that also improve staging and even upper-mid/treble presence. Rel uses high-level connection directly from the amp and proper integration requires careful matching of gain and crossover. There is no app or DSP although you could use REW to measure the room response. Integrating two subs for two channel music is no small feat and most people don’t get it right (at least the ones that I’ve heard). REL is a great sub for music but there are also others that are as good (rhythmic for one). Start with one and see how you like it.
@verendus He’s about as fake a reviewer as you can find anywhere! His camera/lens “reviews” are atrocious/hilarious/criminal!
Nevertheless, the referenced article is apropos.
Do you have a quibble with its content?
I stopped reading his content ages ago. Maybe I should give him a fair chance to see if he actually reviews real stuff. See my comment on one vs two sub for 2 channel. At least it’s a personal experience with the actual hardware being discussed. 
I don’t share your experience/opinion in this specific regard.
Good input otherwise.
Regards.
I’ve heard the 802D2. I thought they had pretty nice bass. How big is that room? I’d assume the classe monos would drive them fine?
ok, but then the post 2 down from here seems to say the opposite?
Sorry, I don’t follow/understand your comment.
Care to clarify…?
One video specifies high output, the other video discusses low output
Gotcha.
I would note that REL allows for both approaches with some of their models, but not all; and, in the context of a HT installation, I believe one can choose to use both connections and activate one or the other depending on whether the signal is HT LFE or music.
A REL T9x or S/5 would pair well with your B&W 802D for seamless bass. Start with placement near the front or in a corner, and experiment for the best integration. REL’s “High Level” input helps it blend naturally with your speakers.