Hi @rower30. So I wandered over to listen to my system (currently wired with Series I OFE on the lower and the Series II TPC on top - which Bob is kindly letting me hold on to until my audio buddies can finally make it over this coming weekend to listen). I sat there and said “WTF? There’s no low end,”. I played a number of tracks, using all my sources. Did the big red button reset on everything more than once. I even pulled out TrueRTA and ran some pink noise, measuring each channel individually. It was pretty definitive. I first thought the lowers on both my speakers were blown, but I don’t play any low frequency square wave type stuff, and never listen past about 80dBA anyway, so I figured that was unlikely. Then I thought both (!) of my Pass amps had their left hand binding post outputs open somehow (I use the left on each for the LF cables), but the Pass outputs are just paralleled hardwired from the output board. So also unlikely. But just for grins I measured the RMS voltage from both sets of outputs on both amps, using pink noise. They were all approximately equal.
I shut the amps down and on the left channel moved the OFE to the same posts being used by the Series II. Turned that amp back on. Everything was as it should be. So I took out the Fluke DMM and measured the resistance from the OFE on the right channel amp, from where they met the binding posts on the amp to where they met the binding posts on the speakers. The positive lead was open circuited. I loosened the binding posts on the amp, pulled those OFE ends, and gave them a bit of flex. Put them back on the same (their own) binding posts on that amp. Checked continuity. Suddenly all good. Turned on that amp. All good.
The likelihood that two of anything in an audio system should exhibit the same odd behavior - at the same time - is virtually impossible. But I have nothing else that is suggesting anything to me. My Series I OFEs are not the later build with the welded ends, if that might offer up anything. Things seem to be working fine now. 
Even though the frequency range is back, the low end doesn’t sound like it did before. It’s incoherent, plodding. I put my other cables back in play, and while they were previously bested by the OFE I / TPC II combo, now the low end out of my speakers sounds like what my speakers are capable of doing.