I use mine with a 2+2 horn system. It replaced an Aesthetix Calypso preamp and a Gallo Ref 3 subwoofer amp and XO. I run my CD player into the analog inputs rather than use the TDAi as a DAC, but it's close either way. I could sell my CD (Aero Capitole II) and pick up an inexpensive transport and be quite happy (and bank some cash).
So I have the TDAi's digital amp driving bass modules with its XO set for low pass, and high pass on the line-outs to a pair of SET mono's. Its delay function allows me to use the subs against the rear wall (or in corners if you prefer).
The magic is when you switch-in room correction.
A nice versatile tool blending old(SET) and new(Class D) technologies quite seamlessly.
Though I only use the amp for bass duties, it is a very good sounding amp. I used it briefly on the horns and it sounded excellent, without the annoying top-end that I'm sensitive to with other digital amps that I've heard. I was amazed that it was dead quiet into a 106db loudspeaker!
I've heard the Lyngdorf DP1 speakers and sub system which is very nice too, but not quite where I wanted to be.
Considering what gear you can replace with the TDAi and how good it sounds, its a no-brainer, which is possibly one of the reasons they don't show up in the classifieds too often.
I've owned Tact gear and have a lot of respect for what they do, but I prefer the Lyngdorf approach to room correction. Tact always had me wondering if I'd hit the right spot with setting the correction curve, resulting in endless tweaking. Lyngdorf just do it for you - an hour out of the box and you're done.
It'll keep me happy, for a little while at least :)