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

It's been some time since I owned any HSU subs, so maybe they have gotten better, but I like the build quality and sound of Rythmik and SVS subs a lot more than HSU.  Consider getting two F12s vs. a single Rythmik sub or push your budget a little and get 2 F15HP subs from Rythmik.  I would also take a look at a pair of SVS SB3000 subs.  Right at your budget point and an excellent value.  Or get 4 SB-1000s and make a Swarm!

I own HSU subs and Vandersteen.
The HSU's are very good for the money. The Vandy's, with their crossovers, are, in my opinion, the best- easy integration, and built like tanks, unlike many brands. ( I would do a search here on Audiogon. I remember one brand was using rather inferior parts in their subs).
I bought both of my subs used, as pairs, so check HifiShark like I did.
I will be selling my 2wq's soon, so I can buy the new Sub 3's.

Multiple subs are the way to go, as they reduce room nodes, There is also the DEBRA that Audiokinesis sells. Ralph Karsten (Atmasphere) finds it to be very good, as well.
Bob

When I was looking into a second sub, REL themselves told me that getting identical subs was important. Others here have agreed and disagreed with that view. 

Via Audioholics: "Ideally you should use identical subwoofers for the best overall performance, but it is possible to mix and match subwoofer brands and types if you're willing to do the extra work. If using mismatched subs, bear in mind that systems with different low frequency cutoffs may well be in phase and additive over most of their band, but at or below system resonance may well be out of phase. This can put us in the position of having the sub with a higher cut-off frequency reducing overall system output below its cutoff frequency.   This is why we usually recommend using identical subs all around."

I asked Brian at Ryhthmik about the F12 and he didn’t think they have enough output to match the cornwalls in my room.

Oz

I’d start with a single sub assuming that the subs used if two are not as good or well matched to the system. Getting the right sub to match is paramount. Best to start with one. That’s all you need if listening from a single sweet spot. Then, if needed to balance out other parts of the room, add more later. I’d stick to a single sub model that matches well though different models might work as long as all match similarly well but I would get one right first then add more of same if needed.

One question I have is how are people connecting multiple subs to their two channel systems not necessarily designed to output to multiple subwoofers out of the box in a manner that maintains proper load?