Straight up comparison REL T7 subwoofer vs Sunfire SDS 12 woofer. Preferences or opinions?

For what it's worth, I just, um, "subbed" out a used Sunfire junior, which I'd thought was pretty good, for a new REL T9x, and it is night and day.  Using with gen 1 Raidho monitors and a Roksan amp.  The REL has completely changed the quality of the sound.  Richer, more precise, much better sound stage and virtually no boominess.  (The remaining boom will be fixed with some isolation feet.) I, too, was skeptical about whether investing $$$ in the REL would be worth it.  There's just no comparison to what admittedly was a pretty budget but still good Sunfire.

stereoisomer

As above, a REL is precise, richer and improved sound-stage. The Sunfire is a close 2nd runner-up. If placement is a preference, go with the Sunfire. It is a bargain and not picky with placement.

Happy Listening!

Don't swap, daisy chain them together using the variable controlled Sunfire as the master using the low level RCA outputs on your preamp.  

A crawl test is worth the effort. Inexpensive custom length cables from Blue Jeans or Monoprice.