I wanted to thank everybody who contributed to help me troubleshoot this. Duck_arse and anotherjim in particular, thanks for your advice. I actually learned a lot from you all, and it changed my practice for the better, even though I still have a lot of questions.
Here is what I think is the final analysis: it's just a freaky circuit.
Built the bass, "correct" version 1.1, same squeal with drive maxed and the last 10% of the treble pot.
Then, I went back to my pcb guitarmania version of the beta with the same conditions, listened hard, and.. it was real close to outright squeal, but stayed in the background. I had subbed TL072s for all of the RC4558s, and so, hey, I subbed all the 4558s back. And... same conditions, same exact squeal. So, three different builds, three different component lists, two different values, two different PCBs, it all points to one conclusion to me: the original circuit (or original circuit mapping) being funky. And it did (and does) behave pretty strangely when putting it through its testing paces.
BTW, as a preamp, the TL072 in the beta sounds really amazing, particularly in the clean and light overdrive range and puts out A LOT of signal. But the overdrive is not as guttural as the RC4558s, which really nail the tone of the original overdrive sound.
My best layman's guess is that something in the TL072s has more capacity than the 4558s, and that the 4558s in the circuit are just maxed out and can't cope with the signal and gain wide open. So maybe the overtopping gets thrown back in the mix somehow and creates the feedback?
And, I did 2 more Aion builds (Binary fuzz and elysium delay), and they worked flawlessly off the first try.
Thank you folks!