No 4 ohm loads on these amplifiers.
The service manual clearly states A or B speakers 8-16 ohms, A + B speakers 16 ohms
The only audio spec differences between the "H" model of the BX-120 and all the other models is the input sensitivity of 1v for rated power which is 75w + 75w @ 8ohms all models, and it also clearly states the additional power rating of 100w + 100w 10%thd @8ohms for the BX-120H................... i'm not sure what the mystery is.
To be clear, it means when you try to drive it beyond rated power the distortion goes way up, pretty much like every other amplifier ever made.
Factor that 100 watt distortion rating with the fact it is not rated to handle a 4 ohm load in any way, and what you have is an amplifier that won't take kindly to being pushed beyond it's limits.
The other models of BX-120 have an 800mv input sensitivity for rated power(for those wondering).
For all models of BX-120 the FTC rule U mode rating is 75 watts per channel Min. RMS @ 8 ohms, from 20 hz- 20 khz, with no more than 0.02% Total Harmonic Distortion.
BX-120U AC 120volts 60 hz
BX-120G, K AC 240 volts 50/60 hz
BX-120H AC 120/220/240 volts switchable 50/60 hz
BX-120E, Z AC 220 volts 50/60 hz