Hey all. Sooooo ... I really had been quite thorough in my earlier troubleshooting, trying different instruments/outputs/amps/levels in the chain, everything suggested to me via line6 support tickets, even going to a store to test another HD 500 with other guitars. Nothing was working.
I'm managing now to dial out the clipping sound. It's still very easy to dial it in, and not entirely clear where it's being introduced. On the Blackface DBL with gain up I need to keep the bass low and switch to 4038 ribbon mic.
I'm still not sold on the unit, so far it's wasted a LOT of my time, I don't think it should be this difficult to achieve a workable tone - but, those of you who said it was possible to resolve that clipping sound with proper settings seemed to be correct.
I'm still not sure that it's a matter of gain staging, as if I leave the bass up even above 25% and leave the default mic selected, I can't dial out the clipping without lowering the master and/or drive to the point of zero or near zero distortion. I mean, you can introduce the same digital clipping sound via poor gain staging, of course, but so far what works to dial it out seems somewhat random, relating to EQ and cabinet/mic selection regardless of gain levels.
I'm still not convinced it's all a matter of accurate reproduction of the original amps, but whatever the case, if it's digital clipping, why is there nothing in the user interface to show where it's occurring in the chain? The software should be able to detect that and the user interface should display it.