So it would seem the cartridge is outputting a signal from both channels, and the left channel signal is apparently getting lost somewhere later on down the chain.
Let me just reconfirm what you've tried here - At least *two* entirely and completely *different*: sets of headshell leads; tonearms and their internal tonearm wires; turntables; TT lead-out interconnects; phonostages; interstage interconnects; and control preamps - with only the cartridge (which tests fully operational) remaining the same before the power amp and it's lead-in interconnects, and with other sources such as CD playing correctly in both channels throughout all of the substitutions (thereby ruling out faults anywhere after the control preamps)? And there is no common step-up transformer, or junction box or adaptor jacks of any type in use before the phonostages?
In other words, *every single thing* in between the cartridge's pins and the interconnects going to the amp has been replaced with no changes to the left channel status, and both the cartridge and everything after the preamps' output jacks is proven to be working correctly as far as you can test? And for every one of the substitutions, there was no interruption of the right channel status that would indicate the insertion of a faulty piece of gear?
If all of the preceeding is so, then I suppose you cannot rule out the small possibility that both of your phono chains have different pieces of gear in them with the left channel out. I would go back and reinstate your normal chain of gear you were using when the problem first appeared, and then methodically go through it swapping each connection right channel for left channel one at a time in sequence and listening each time to see if the problem switches channels at the speakers, beginning with doing the headshell lead channel-swap once again, and doing it first at the cartridge end and then again at the tonearm end, and then so on down the chain toward the preamp (at each step, you can either leave the previous connection reversed for the moment if nothing changes or put it back - it shouldn't matter if the problem channel doesn't switch sides with any given connection channel-swap), and see if the dead channel still refuses to switch sides until you get to the interconnects leading out of the preamp (and if that's what happens, double-check yourself by then plugging your CD into the *same* preamp inputs you're using for the phono, just to make sure it results in sound in both channels again). But presumably, at some point along the phono chain the dead channel will have to switch sides at the speakers during this process, indicating where the fault lies. If that happens, then I would re-hook-up the other phono chain and do the same thing all over again, to see where a possible similar fault lies there.
It's a bitch, but there's got to be a rational and discoverable explanation, however unlikely, for what's going on here...otherwise, I'm going to start believing in audio-gremlins, and I might have to run out and purchase an entire Shun Mook suite in an attempt to keep them at bay! :-)