Hey, thanks for wading through this stuff. Some comments:
"- Ohms and the OT --> how low could you run the Twin's ohms without frying the OT? Jerry had a lot of speakers and different configurations and IIRC the OT on the Twin expected 8 ohms."
The Twin expects to run at 4 ohms. With a direct box/power brake kind of thing (mine converts speaker to line level, switchable 4/8 ohms, provides a dummy load to the amp, and has an output level pot) that's the load the Twin sees.
"- Full volume on the guitar sounds great but can add to the equation to overdriving the poweramp. Depending on the setup of the gain stages it is very possible to drive duals into a preamp at full guitar volume and not begin to clip the preamp BUT that high signal level could clip the poweramp."
Yes, I see your point here, if I overdrive the preamp I'm also overdriving the Twin's power amp, with no independent control over to what degree the power amp will clip (I'd have to turn down the preamp input levels to make the Twin's power amp clip less). That's not so bad though, just not perfect.
Did Jerry generally run his Twin preamp clean or did he crank it? Probably it depended on the context, I don't know though, maybe you do. If he wanted to overdrive the preamp for a particular sound that it gave him, he could do so without also adding clipping from the Twin's power amp (since he wasn't using it).
On the other hand, he didn't have any power tubes, and I've always thought of having power tubes as a good thing. That's partly why I opened this issue was to see what people think of that, since it's entirely possible to play loud and clean if the power tubes are used not to drive speakers but to add tone to a signal that eventually goes to a solid state power amp. Is that squishy tube amp feeling because I'm using the power tubes instead of just the preamp?
"- Direct boxes that convert a speaker level signal to a line level signal are not horribly reliable from what I hear and I'm not quite certain they were around then."
This could also be part of it, I don't know. Mine is pretty good, although there is some rare (very rare) thing that happens when I'm playing at high volume where I'll get an odd feedback that I haven't been able to figure out, and it may be related to "the box".
I was just reading the preamp out mod description on Dozin's site. It says you lose the vibrato, what about the reverb? It looks to me like the reverb is still in there, is that true? If so, did Jerry have one of those "dwell" pots so he could crank the preamp without overdriving the reverb? So many questions...