a subwoofer choise - dual 8" SVS or one off-brand 15"

Don't mix pro subs with home subs, they have different goals.

But... two 8" drivers have about the same surface area as a single 10", so it's not a really deep sub.  Kind of depends on the room and what you need.

I'd skip both and look for a decent 12" to 15" home sub, especially if it has built in EQ/DSP capabilities.

i forgot to add that this will be a second sub with a JBL 310 ... not a great sub

I like the SVS.  It EQs to 23 Hz. Much easier to tune to the room with the app

Big +1 @erik_squires who’s giving you very wise advice.  Go for a pair of SB1000 Pros that wouldn’t be much more expensive than only one 3000 Micro and aren’t much bigger but definitely much better and more capable subs.  Sell the subpar JBL and the price differential shrinks further still.  And you’ll now have the ability to effortlessly dial in both subs with the excellent SVS app.  No brainer IMHO, and best of luck.

https://www.svsound.com/products/sb-1000-pro-subwoofer