Rythmik or REL Subs to pair with Totem Element Metal V2?

I have the Metal V2's, although in a smaller dedicated room. Have you looked into Totem's Tribe Solution Sub? Years ago I had a Rel subwoofer.I wasn't too impressed with it. If I was in the market I would look at Rhythmic.

I can't speak to your particular room setup, or exactly what you're looking for.....but absolutely check out Rythmik. I have a G22 (dual opposed 12") and the thing is an absolute beast. Very musical and comparatively easy to integrate. JM2CW.

I’m going to echo @hilde45 for several reasons. I have zero experience with REL, but appreciate that they are well regarded. I use Avantgarde horns (Duos) and had several old Velodynes (an 18" and a 15") that were part of a large home theatre projection system. I could not get them to cohere with the horns for the two channel audio system. I bought one Rythmik 12" (paper cone) and got some sense of what I could accomplish. When I moved from NY to Texas, I bought two 15" woofers from Rhythmik (those do not have the paper cone option or at least did not at the time). I added a cheap DSP unit that only affects the subwoofers and do not roll off the main system integrated woofers. The end result is seamless, very coherent and extremely cost effective. Dialing in phase/delay makes a significant difference in my estimation:

 Dual stereo Rythmik stereo F12SE with paper GR Research drivers. I agree with others here. The integration with your speakers can be more flexible and I agree the output is not extremely loud but accurate. I think REL makes good subs as well though.