Subs for Treo CT

No crossover. Just add subs. Buy more, not bigger. Three pretty good subs is better than one really good sub, in other words. The key to really good bass is lots of subs spread around in different locations so that they all have different bass modes at different frequencies and locations. This results in much smoother bass than you can get from fewer subs. Do a search for Swarm subwoofer system or distributed bass array, and pay attention to the guys like me and Tim who actually have done this. https://systems.audiogon.com/systems/8367 

Notice this is five subs, three different types, in random locations around the room. The bass is phenomenal! And Moabs already have really good bass, but with DBA its even better. 

Listening at moderate volume you will be able to adjust yours to the level that works best for the volume you care about most. It will be sort of like a loudness control. This won't work nearly as well with one or two subs because the bass is lumpy but with a DBA its so much smoother, so much more articulate and tuneful you'll be able to set it higher which will be perfect for lower level listening. If you turn it up real loud you will have too much, and in the long run you will find the happy balance. 

You won't need bigger right now but long term might be a different story. Notice mine are mostly 10" and my room is big 17x24 so its not like you need big subs, but if you can get a deal or will be in a bigger room some day then maybe buy for the long term. Main thing is to get to where you've got at least four. 

Great choice on the Treo CT I'm abig fan of Vandersteens have Quatro woods  your best sub with the Trees is Vandy sub 3 it can be dialed in to the room and blend perfectly 
the cross overs work by cutting the lows 6 Dbs to the Treos and the sub 3 take care of the lows hope that helps a little if you need more info contact John at Audio Connetion he a fountain of info 

Enjoy the Music 
Tom

@Tomstruck has it right - try to get a Vandersteen sub 3, you will get accurate bass and take major load off Treo and importantly the Treo amplifier with the high pass system. Swarm is competitive and an excellent solution in situations where you don’t mind 4 more boxes and want better bass in more areas of the room. By now you probably understand that the Treo are designed for an optimal seated listener position and the 11 bands of EQ on a sub 3 enable you to get great bass at that position.
Have fun, enjoy the music.
who is your local Vandy dealer? I know most of them.
jim

Congrats on the Treo. IF you need setup assistance don’t hesitate to contact me - I do that frequently- just finished a pair of Treo CT with Aesthetix Mimas integrates - lovely sound.
jim

@OP,
Trust Tomic, he knows Vandy’s well.
If I were you, and if money is a concern, then I would get a pair of Vandy 2wq subs. You wouldn’t get the equalized bass, but you would be pretty close to a Quatro.
REL subs are good, but not in the same league as Vandersteen.
My main system has Treo’s (non CT) with the 2wq’s, my office has VLR’s with HSU subs.
As far as integration goes, the Vandy subs win, hands down.
They are also built like tanks. I owned a pair of 2w’s before the 2wq’s and they must have been 30 years old, yet they still functioned like new.

My next move is to sell my 2wq’s and get a pair of Sub 3’s.
If you are on the East Coast (or even if you aren't), Johnny Rutan is the man to consult. PM audioconnection.
Bob

Jim, Thanks for the DM. Think I will answer here  because the answer is great general info most people still don't know about, because it runs against the CW.  

All rooms are small relative to the wavelength of the lowest frequencies we want to hear. Those waves are 50 feet long. For those you need a room no smaller than a hundred feet or so in any direction. Any normal house size room the leading edge of the lowest bass has hit floor, ceiling, and all four walls and all the way back to the sub before the first wave cycle completes. That's small.  

Its these reflections that create the problem, and its multiple subs that solves the problem, therefore the worse the problem the more the better. So its sort of the opposite of the CW, and the smaller the room the more you need DBA and the better it will work. They don't need to be as big, but they do need to be more like four.  

True, many subs can be EQ'd for very smooth deep response at the one spot its EQ'd for. Here's the problem. A big factor in muddy bass is that bass energizes the room. This takes time to dissipate all during which its resonating back into and muddying the music. So let's say you are down 5dB and so you EQ up 5dB to get flat at your one spot. Okay. But that 5dB is still coming out the sub energizing the whole room. Muddy bass. CW says no sweat, here buy these tube traps. Wonderful wonderful tube traps! And yes you can with enough stuff get it to sound pretty good. At that one spot. With all this extra stuff.  

DBA says hey you are down 5dB no problem don't touch that sub just add another one. And put it where its not down 5dB. Or where it together with the first are not down 5dB. Now you got it flat and not by adding bass but actually taking it away, because now with 2 subs you get to drop the volume of each one to get back where you were. Do this again and again, four subs, now each one is much less volume but all together are as flat or flatter than the EQ'd setup, only we're putting much less total energy into the room therefore its way faster more articulate sounding.  

This all applies only more and more as the room gets smaller. The main difference is you might be able to get away with smaller subs. But I would not go smaller than 10" unless the room was really small or they had to be hidden behind or under a sofa or something like that. Four 10" subs works remarkably well in a wide range of room sizes from yours or smaller to mine or bigger.

Thanks, very helpful!