Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

for 2 channel stereo listening -- it isn’t about the amount of bass

it is about distribution of the bass and how it matches/mates w your speakers and room

two usually work MUCH better than one if the room is of any meaningful size

suggest you consider older, relatively inexpensive Rels... use high level connections - should be able to get two for just around a grand

Well that's the trick. Finding two of anything used at relatively the same time isn't easy.

violins playing in the background... :)

Yep,patience  is a virtue.  You have to keep scouring for sale ads. Try hifi shark. I believe you can do a search and get alerts.

Also consider the possibility that "upgraditis" could set in if you go with the 2 lower priced/lower performance subwoofers especially if the higher performance/priced sub was within your financial reach. In that case, it would be wiser to get a single more expensive sub first as it would save you cash in the long run. After you get the 1st sub, skip a burger a day and save the cash for the 2nd one 6 months later. It does require patience.

If you feel that you won't be affected by "upgraditis" too soon, you could go with the dual HSUs. "Upgraditis" is a common disease among audiophiles (reason for this comment).