Subwoofer for music, how low?

Virtually no music is down in the 20-40 hz range and I doubt the t9 can really deliver there anyway. I have a REL S5/SHO and properly set up in an acoustically tight 13x 22 listening room it does a great job of delivering deep bass- when I measured there just isn't much music at those very deep levels. Most sub activity is in the 40-70 HZ rang or so. I'd go with a larger box and drivers which will provide deeper and louder bass than the T9. 

I have a Totem Thunder, with my Forests but just ordered a pair of SVS sb-1000 pro subs. I’m not in need of excessive bass, especially with the Forests that have a considerable amount of bass already. I wanted a matching pair to give flexibility in my small/medium size room. They have a sale going on right now, so I went with the pro series as it has an app for my phone that can control the many features of them. Nice to be able to control them without trying to “access” the controls.

If you can afford JL Audio, I would go with them. I’m utilizing mine with the Wilson Audio speakers and the Wilson Crossover. Easy to blend in the room and you don’t get wasted sloppy bass. It is very precise and accurate down to 20hz if any of your listing music goes that low. One of the best subwoofers around and a great reputation too boot.

I am also utilizing 4 JL Audio Subs. 2 Fathom 113’s and 2 Fathom 110’s

Maybe I missed, but didn't see the size of your audio room. With base, the size and shape of room, along with budget, will determine what to do - one good sub or a swarm of subs. 

I have a medium/large room (14' x 27'' open to an additional 11'x13' adjoining room) and a single ADS Sub loads the room from 60Hz down to 20Hz incredibly well. In general, the smaller the room the more need of multiple subs to load the room with convincingly deep base. 

If your room is a bit smaller - say 12'x20' (3.5m x 6.5m) for deep full base I would think you could do very well with a pair of good subs that will go down to 20 or 23 Hz (you might check the used market) and set them to operate below 60Hz.

Keep in mind, quick and punchy base is not deep base. Quick and punchy base (at and above 60Hz) and deep base, getting down near 20Hz, should integrate and augment each other and even after getting the right subs - that will take some time, work and patience.

Best of luck......Jim