The HXDA is only based on the Plexi. It's a single channel amp. There's no Fender in it, and there wasn't supposed to be. I've had one since the model came out.
HXDA backstory:
PRS got hold of Duane Allman's original Plexi Superbass that he actually used on 'Live at the Fillmore' and cloned it. While creating it, they discovered that with a few switches, they could switch in parts to make it a Superlead like Hendrix' (I believe they got hold of one of Eric Johnson's Superleads). Thus, HX (Hendrix) and DA (Duane Allman). It absolutely nails both. It's a very authentic sounding, vintage sounding amp that does late '60s Marshall beautifully. It isn't dry and grainy like the later JCM 800s, etc. There's a liquid quality to the tone.
The DG30 was conceived as a blend of Grissom's favorite attributes of his vintage Tweed, AC30 and HiWatt amps. It's a stellar and unique amp and shares the #1 spot in my studio with the HXDA. My feeling is that it sometimes leans Tweed, but has more thickness in the mids, the bass holds up better, and it's fatter than an AC30, too.The tone stack is said to be influenced by the Hiwatt. After experiencing the HXDA, I got the DG30 when it was introduced. It's phenomenal. It also has a boost circuit that can be switched in that alters how the amp responds, and a hi-cut control like the ones on the AC30s. You can do a lot with it.
MDT was Marshall, Dumble, Tweed, not Twin. But as with the DGT it's really its own thing. Based on a short listen, I thought it leaned Marshall more than Tweed or Dumble. But of course, the original Marshalls were copies of the Fender Tweed circuits. Dumbles are in a different place. I don't really hear Dumble in the MDT (was a Two Rock player for a long time, so I'm pretty used to the Dumble style sound).
I should mention that I have some other nice amps, but when I play through the HXDA and DG30, I feel like I'm playing amps on a different level entirely.