I don't know about the dbx I & II differences causing a problem - it seems that they might - BUT, I think the more important issue would be head allignment and proper track playback. I contacted TASCAM a while back asking a similar question, and I was told at the time that my Porta One masters would only play back on a Porta One, the reason being that they used the head allignment for the Porta One only once. On their futute models, the allignment changed.
I think if you have masters from a more current machine, you might be able to pop 'em into a different, (also) more current model and get good results, but the Porta One is an exception.
At least that is what some dude at TASCAM told me.