Calamity!
I closely examined both amps together for the first time, and got a rude surprise. Let it be a lesson to all of you in getting too excited before you get your hands dirty.
I knew that there were substantial differences in circuit between Left and Right amps, but I was dismayed to see that the choke on the Left amp is noticeably smaller. The printing on the power transformer is also more of a forest green than the blue printing on the Right amp, though I don't know if this means it differs internally.
I've scrutinized photos of the M6, and the transformers and choke appear to be identical on both channels. I am unsure as to whether this is true for the earlier M5 I got these amps from, or whether the discrepancy is due to a running change in production and bad luck on my part.
In any case, I think I'm going to mothball this project, in hopes of eventually putting together a pair with matching iron.
Never fear, though. This copper-chassis Motorola single-ended EL84 amp showed up at my doorstep last week. Should be an easier project, being a stereo amp as opposed to dual mono, and already rocking a 315 V secondary and 6CA4 rectifier. I'm making plans for a wood frame, filter choke(s?), banana jacks for speakers, and a metal plate (Front Panel Express?) to be mounted on the forward section of the amp to hold 12AT7 socket, L & R RCA jacks, and dual DACT-type attenuators. Cost of those additional parts is mostly offset vs. the Akai amps in half as many power supply caps and no need for tag/turret boards. Hopefully, it won't take me long to get started, as I'm hot to make some progress.
The Akai OPTs still look a little beefier than the ones on the Motorola. I understand that both are 5K at 8 ohms. Wonder if I should "borrow" the outputs from the Akai amps. I'll almost certainly be stripping the Motorola down to the bare chassis before rebuilding anyway...