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

Thank you for your thoughtful and intelligent reply. This is exactly what I was looking for.

If the driver fires downward against an attached bottom, that bottom is a butt plate.
A better idea is measure the slot height and width, just like REL does and attach a 3/4-2" bottom that distance from the bottom of the sub cabinet. "Slot load" the passive radiator. It can be open front, back, or side, but not DOWN.

THEN you decouple the butt plate or slot from the floor with Springs, Pods, or Air ride. But you NEVER EVER couple anything to a domicile structure.

Think of the Bay Bridge, How much steal would it take to span the bay with just steel, from terminal to terminal all the way across the bay, how THICK would it have to be?

Now think of the springs (cables) they used to suspend the road structure? Remember what part of the bridge that failed during the earthquake? The ridged top deck failed and FELL on the suspended structure below it. SPRINGS? You cannot make some things strong enough on a planet that vibrates. The bolts sheered, they broke bolts in half that were as big as my arm. The springs held..

BUT I suppose you can keep trying.

To heavy of a spring and not enough mass "Galloping Gertie"

Regards

http://pointillistic.com/vmps-audio/subwoofers.htm

You see that slot on the bottom of that column. There is a 15" high excursion passive radiator in there. You adjust the columns Q with putty pinching a fingernail at a time. Very accurate way to mechanically tune to your room, your amp, your cables, your ears.. To bad the company shut down in 2012. The Ideas didn’t

He was getting away from selling that type of open slot (RELs too) on all his subs..

He use front firing slot loaded with a bottom plate on all his floor standing speakers from the 70s on..

Regards

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.