The videos pretty thoroughly cover the territory; there was a terrific demo early on in Paul's studio that sounded so good that after watching it, I immediately ordered one.
I think the amp's operation is pretty straightforward; there was a sheet (not really an owner's manual) on the PRS website re: the 25th Anniversary amp, upon which the HXDA is based. It might still be available for download. Most of the info applies, except that the wattage switch was changed to a master volume control, and there weren't the switches for input, tone stack, and cathode follower.
Because Tim Pierce's demo of the amp shows off how well the amps respond to the guitar's volume control, and how good it sounds at higher volumes, I think it's a great source of info on the amp's possibilities.
Really, it's just great to experiment with an amp and find whatever for you is its sweet spot. I have some Mesa amps, and they have excellent owner's manuals, but sometimes I think that it's great just to turn the controls and see what happens!
True!
@Tahlee added a copy of the sheet from the 25th Anniversary model near the beginning of this thread. I added a copy to my records for the HX/DA. I'd love to get a schematic as well, but haven't found anything more in depth than the tube chart. The schematics exist... but whether it can be viewed by mere mortals is another matter!
A few more demos:
HX/DA 50 and 2x12 Cabinet:
Sweetwater Demo w/ Mike Clark:
Paul and Mike Ault/Nicky Moroch with the HX/DA:
Points that Paul makes here, interpreted by a person that can't be sued for saying Duane Allman, Jimi Hendrix, Allman Brothers, or Marshall:
HX/DA Info
This amplifier was cloned from Duane Allman’s Marshall amplifier, which was used on the “Live at the Fillmore” recording. This amplifier, on inspection by PRS and Doug Sewell, turned out to be more a Marshall bass amp circuit than the expected normal Plexi. The HX/DA, with all switches in the DA setting, is this amplifier.
Jimi Hendrix was using Marshall Super Lead 100 (1959 Plexi model) amps in the period following this, which are a modification of the circuit encapsulated in the DA settings. The HX settings make this conversion when all activated. You can use any combination to explore sound possibilities.
Switch 1- Moving this to the HX setting takes some of the bass out of the first stage, which begins to take it towards the brighter sounds of the Super Lead amp.
Switch 2- Moving this to the HX setting adds some gain, which Paul points out is in the cathode follower.
Switch 3- Moving this to the HX setting changes the feedback resistor, another modification that Marshall made in creating the Super Lead Plexi used by Hendrix.
And Part 2:
A great 30 watt demo showing how volume and picking on the guitar end can make tons of variation in drive on the amp, from Wilcutt Guitars:
and His Grissomness demoing at NAMM 2013: