Thanks so much for the response. I'm totally into all the new tech and the higher-quality HD modeling - there's definitely stuff there that the X3 can't match, and that's why I bought the HD and I use it. It's just that I have some patches that create a wall of sound on bass (using for instance FX Seismic Synth into 2001 Treadplate Dual on Tone 1, and FX Double Bass into Line 6 Insane on Tone 2) that we've recorded with, and I need that exact sound. Not "newer" or "better", just that sound.
I feel like it's like telling Trent Reznor that some crusty old 8-bit sample from a 90s platinum-selling album won't play on the new 32-bit sampler, but he's welcome to re-create it from scratch. He doesn't need the old 8-bit sample to sound 32-bit, it just needs to play back. :)
So yes, I'll have to roll up my sleeves to try to figure out where to start on recreating that patch, second-guessing myself all the while, and wondering if all my work will be useless when the next POD gets released with different "better" amps and effects.
So I guess let me ask - what are the closest equivalents to the Seismic Synth and the Double Bass in the POD HD?