I’ve owned a few Rel subs up to the S/5 level. I currently own an SVS SB3000 and like it better. I like deep bass and this sounds great with my setup. Would I like to hear the Rel S/812? Sure.
If your amp doesn’t have the sub out (many don’t) you will need something like the Russound speaker-to-line-level adapter to make things work. Compare these you mentioned to the Rythmik F12G which doesn’t have the app that the SVS has but "might" be better. I once had both and it was close but I like the phone app.
Good luck with your search. To answer your other question Rel has- great marketing.
If your amp doesn’t have the sub out (many don’t) you will need something like the Russound speaker-to-line-level adapter to make things work. Compare these you mentioned to the Rythmik F12G which doesn’t have the app that the SVS has but "might" be better. I once had both and it was close but I like the phone app.
Good luck with your search. To answer your other question Rel has- great marketing.