Just IMO that wouldn't even be comparing apples and oranges, that's like apples and Slim-Jims or something. If you think you need 500 watts, and if you think just a tube preamp will get all the tube tone you want, why wouldn't you just go with a small high powered class D head and find a tube preamp pedal for that part? The whole reason for dealing with the inconvenience of the size and weight of a 100T or SB300 is the tube AMP, and the flexibility of switching between tube or solid state preamp sections to drive it. I chased my tail for 4 decades trying to achieve the sound I wanted with tube pedals or preamps paired with solid state amps. After making a WAG and getting a 100T, it turned out the sound I was chasing all along was the sound of a tube amp, not a tube preamp. And in my case the sound I was wanting was a tube amp right on the edge of stressing out, which the 100T does beautifully without being so loud you're blowing doors, especially if you run it at 25 watts, which I do almost exclusively now.
May I ask what you mean by : "After making a WAG..." ?