The Dr. Z's are very basic, kind of a one trick pony. Reminescent of the first generation of Matchless just not as hot. The one trick they do is very good, but not versatile. with a crap load of pedals in front it may be a different story.
As for the THD...I've tried most everything they make and I've never been able to dial in a good useable tone. Be it Bi-Valve, Uni-Valve, swappin' tube like mad, I didn't like the voicing.
I'm to the point that if i can't get a great tone with just my guitar straight to the amp...I'm not going to bother. Look to Fuchs or the new Supro's made by Bruce Zinky. He was head of Fender's Custom Amps a few years back. he is responsible for Tone King's Vibro King's etc... I have played his new Blue Velvet's and they ROCK. He also has a 3-10" 50W version of his Vibro King in a Supro.
Zinky Amps