I've read your great article on ETM but if you have any other gems of advice or tweaks - now you have the benefit of experience and hindsight - then i'd love to hear it!
I have not really had any desire to tweak them. About a year ago I lined the cabinets with No-Rez from GR Research. This made things a bit cleaner, probably because the cabinets became somewhat less resonant, or at least the resonant frequency changed for the better, whatever it is. I also tried removing the foam Earl supplied from behind the woofer after installing the No-Rez and thought the speakers had a more natural tone on things like a kick drum, perhaps it was a bit over damped with both the No-Rez and Earl's foam.
Since you are using heavy PU I'd think your cabinet resonances would be much less than with the MDF mine are built from so I'd likely go with whatever Earl is suggesting for stuffing.
Since you are building the 12a version, the crossover is a little different. I have not made the upgrade but heard from a friend this week who did. Part of his comment to me was "What I heard was a subtle smoothing of the frequency response around the crossover point most noticeable on the high registers of female vocalists and a small improvement in bass resolution. Again, I would rate the difference as subtle, but I'm pleased I made it.". Given this, I may take the plunge and rebuild my Abbey crossovers to the 12a spec but I'm in no hurry to make any changes as I still enjoy them very much.
mike