@daveyf
It seems we are always destined to do that.
@phusis
No, you correct it as much as you can especially if the irregularity is only in one channel. It is not so important that the amplitude curve is perfectly flat, it is important that the two channels are perfectly identical, or as close as possible.
And just how are you correcting amplitude 1 Hz at a time? It is not down the road for me. You can do it in an automated fashion or manually including programing delays. I start with automated then fine tune manually. I find it best to program for flat then overlay my own preference target curves which were constructed by ear.
Efficiency is nice if you want to use small amplifiers. Personally, I do not care about it. I prefer to look at the type of loudspeaker. You like horns, I prefer ESLs which admittedly are not efficient @ 86 dB. But, since I remove 100 Hz down from them they go louder than ---- , which is all I really care about, the ability of a system to reach realistic volume levels. Back in the day speakers like the EV Patrician and the KlipschHorn were SOTA as the most powerful amp we had was 70 watts/ch. The Marantz Model 9 comes to mind. Then came the hideous Crown Stereo 150 followed by the Fuzzlinear 700, somewhat better than the Crown, but..... These initial SS amps were the reason people stuck resolutely with tubes. Some hangovers are hard to get rid of. At any rate with the amps we have today efficiency is not an issue. It only determines volume per watt and not sound quality.