My second unit, not plagued by bad drivers, sounds ok. My first had two bad drivers, my NOS super tweeter was perfect but my replacement ebay midrange was worse than nothing: scratchy sound (which you can feel, moving the driver by hand). The availability of even decent looking HPM 100 midranges being something like nil, I decided go try an HPM 700 midrange, said to be physically compatible with the hole anyway, and possibly improved or better.
Many tricks here to get a loud playing speaker. The first, as always, is not to actually play loud, such as in the bass below 40Hz, where by 33Hz I'm not even sure if there is even "useable" response--it's there but so far rolled off. Well this bass rolloff means that you don't have to worry about the mega excursions that would otherwise be required to reproduce 30 Hz at 110dB or whatever, let alone 20 Hz. Actually even by the tuning point around 40 Hz there's already quite a bit of rolloff and around there anyway it isn't the speaker moving anymore it's the port.
A modern paper purist would argue plain old paper cones, with their natural damping, would be best of all. True, perhaps, but it wouldn't have that 'monitor' sound.
But I think maybe the midrange could be rethunk, certainly on the first HPM 100 it attempted to cover too far a range, leading to lots of burnout. The A version limited the range to like a few Hz in the midband, that's actually not such a bad solution, the woofer can support higher, just not quite high enough to crossover with the tweeter. A modern high efficiency driver might do the improved narrow band even better.
I'll leave my good unit as reference and the other as testbed for new drivers and crossover ideas. I like the narrow midrange crossover idea actually (contrary to many).