My experience with Adcom amps back in the day was that they are voiced warm like the B&K, but lacked fidelity and slam. They also sounded dirty/grainy/messy. Ultimately, the B&K amps were just so much better.
I don't have experience with the Theta amps, but I think they are going to be in the neutral / uncolored category. Likely to be very excellent. I don't have experience with the nCore Class D circuits, but I have always preferred Class AB amps because they have better extension into the ultra-high frequencies.
The Theta Dreadnaught D can get very expensive. If you are looking for an alternative, take a look at the ATI
AT5XXNC series. They use the same nCore amp circuits and also have linear power supply like the Theta, but these are much cheaper. The Theta looks to have much beefier power supplies, but it can cost double or more $$$.
The Krell TAS (Theater Amplifier Standard) is part of the Krell Evolution series amplifiers. In my opinion, these were the absolute best amplifiers that Krell made. They are very high resolution and very transparent / uncolored. They are not like the newer Chorus/XD line, which are lush and colored. The downside is that they are very old (17-19 years). If you are interested in the TAS, I would consider sending it to Krell to have it refreshed / recapped.