I'll try to keep it simple, but I can only guess as to how the HT Sustortion circuit worked. I just read a thread posted by an HT amp owner who seemed to know a lot about them. He confirmed that the Sustortion circuit generated only even harmonics, so the AirWindows Density plugin may not be a good choice because it generates mostly odd harmonics. Not sure what to recommend for that, but I know the odd harmonics can be somewhat reduced by turning the Bias knob down on the free Nick Crow Lab "Tube Driver" plugin, which also softens the feel. It has great pre and post filtering options as well. Setting the Oversampling to 8x rounds the top off nicely, but it is a CPU hog.
You might find it useful to run the "PurestDrive" plugin b4 the Tube Driver. Its one slider adds an ultra-smooth subtle saturation/sustain that allows focusing the dynamic emphasis b4 running into the Tube Driver:
www.airwindows.com/purestdrive-vst/
You could even add Coils after that for some Neve-like meatiness if you want.
Keep in mind that Alan's guitar pickup was a very high inductance PAF type made by Seymour Duncan, so it likely rolled off below 2kHz depending on his cable capacitance -- my guess is at ~1.6kHz. It's an important element of the sound, along with his picking technique and how the guitar damped the string sound. It may have been his Red Basswood body Charvel, but he had a Spruce body one at one point as well. Probably not a significant tonal difference through the pickup though.