Rythmik F 12 vs. SVS SB12-NSD vs. ?

Maybe the REL T7.

I'm looking to add a sub to Quad 12Ls for analog audio, not home theater, for under 1K in a 30' x 15' room. Also, I'm not sure whether to run the speakers at full range or crossover at 80. Rythmik suggested different amps for each way. Any advice?

Can't really answer your question directly, but when I asked Rythmik about a purely audio only sub they recommended the F12G over the aluminum coned F12.
I did a TON of research a few years back for a pair of subs for a high end, music-only system. That involved auditions of REL, Velodyne, JL, Martin Logan and a few others. It also involved a lot of test bench data review. Based exclusively on the latter, I narrowed it down to Rythmik and SVS (both sold direct, neither available for local audition). In both cases, I was looking at sealed box 12" subs.

These two test off the charts. The SVS had somewhat greater clean output below 40hz and the Rythmik has better group delay in the same band. I chose the Rythmik and have been very happy with the choice. However, I can't offer a comparison as (other than Velodyne SPLs - not in the same league as the Rythmiks), I never did a proper A-B in my listening room.

As to x-over point: That is a controversial subject that is complicated by room, set-up, and electronics.. My own take is:

If your electronics include full-range room correction (i.e. Audyssey, et al), cross as low as you like. If not, but your set-up includes multiple subs, cross higher so that the smooth response from multiple subs provides as much benefit as possible. For a single, non-corrected sub, higher is usually better, but trial and error is required.

Just MHO.

Personally I'd try to run dual subs since you have a good-sized room, and the dual subs can help even out bass frequencies and be run in stereo if that's something you might want. SVS offers a special where you can get two SB12s for $1150, which seems like a whole lotta quality bass for the money. Best of luck.
Two SB12 vs. one Rythmik F12. Hard to decide without hearing any of these.

Marty, what is group delay?

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Thanks for the link. Better said there than I ever could hope for.

As for the audibility of the delta in group delay performance between the sealed Rythmiks (best in class, AFAIK) and the sealed SVS (better than just about all others), who knows? In either case, relative performance on this test is excellent.

As to one Rythmik vs two SVS, that's an easy call (IMHO). If you are NOT using room correction: I'd definitely go with the two SVS. IME, the FR smoothing created by careful placement of two uncorrected subs is likely to overwhelm any performance delta between two well designed models. If you have the room for two, this one's a no-brainer in my book.

OTOH, if you're using room correction, this might be a closer call.