I ran a Dually for a while and was in heaven. Best cab that I ever owned. Was down at Mark Robertson's for a gear nerd day. CL400Peavey's brand spankin new DuallyT was there. He was kind enough to allow me to hear his new baby before he did. We tested side by side with my Dually, using my Reiner 6SL7 Pre and a Carvin DCM 3000L for power. Also ran my my Streamie. We ran two different P's, My Custom Grove (Funeral Bass),my Dingwall Super J5, and yes even Mark's Blast Cult Upright and Epi Upright. Oh and Mark and I both played all of it, having different playing styles.
Both cabs sounded phenomenal. In the end, both Mark and I agreed that the DuallyT, while nothing overt, had something magical going on. And while both cabinets sounded amazing, it was important to me to have the T option. Why wouldn't I, since I can add as much or as little as the material demands, or even kill it all together.
As for slapping, yes it can be done on the Dually and sounds more than respectable. It only failed to accurately reproduce was I was playing one time.......some stutters. And I suspect that it was likely attributed to my technique because I was never able to recreate it again.
With my DuallyT, I can pretty much cut someone's head off with pulls, which tells me the tweet is giving me edge to round out that tone.
I play so often and run two different heads with great tone and response. I basically adjust bottom/lows for a room, then finalize tone by dialing in the amount of tweet I want. It has ends up anywhere between 3 and 7. Never more. Never less. And that's irregardless of the bass I use, or playing technique i.e. Fingerstyle versus picking.
In the end, I felt that the T was valuable enough and additive enough to my taste in tone that I sold my Dually and asked Mark to make me my DuallyT.
My thoughts are this. Unless the choice literally comes down to buying Dually or not running a GB, buy the T.
I am well north of 200 gigs/events on the T. I spend much of my time playing CC/Praise. Depending on material, I may need clean and warm, up to just shy of fuzz. I need a cab that can handle massive transient attacks like grabbing a huge handful of low B to shake both the floor and rafters. Frankly, I have yet to find a genre that the DuallyT cannot handle. I even run my NS NXT upright through it and it sounds amazing.
I havent looked at another big boy cabinet since, nor do I have the desire to.
Here's Mark fondling the Super J5 while we tested Dually versus DuallyT
Mark actually has lots of toys in his studio. We pretty much played through everything in multiple iterations. DuallyT was the best sounding cab there, bar none.
My big boy rig
Both my rigs. That's an F115 next to the DuallyT
Bass Face from Dually
Bass face from DuallyT