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

If it were me and my money, I would get two HSU’s. You will get more even bass. I run three HSU’s in my 9x13 listening room with very good results. 

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.