W0ARM - Alan
I also have a fairly early S/N 101D, which was indicated to be one of those that needed the ALC overshoot correction. This was a hardware modification/correction/fix by Yaesu (not firmware), and is actually a mod which can be done at home, IF you are comfortable working on SMT circuits.
I have run a number of different amps with my radio, (all tube rigs, no S/S) and have no detectable issues. However, that said, I do not use ALC. I limit my drive power manually, and tune correctly, so ALC is really not necessary. The ALC "overshoot" issue was a case of the transceiver not responding quickly enough to the ALC input [from the amplifier] and not clamping/reducing the drive quickly enough, causing the brief over-drive/over-shoot anomaly. No ALC = No problem.