I'm late to this thread, but own both an MF 2500A, & a MF2250 ("non-A"). No hums with either. They're both very musical-sounding amps. I can't tell that the MF2500A sounds "better" than the non-A MF2250, except for the increase in power. And as I recall it's several hundred $$ for the upgrade....
Any way to "improve" a Conrad Johnson MF 2500
I recently purchased a 10 year old CJ MF2500 amplifier. It is in perfect condition and it sounds excellent, but I noticed a hum coming from my speakers when it is turned on. It got me thinking, since there is no way to upgrade the power cord, if there was a way to "refurbish" or update the amp. I don't know if that is something normal for a solid state amp, but it's sounds so good and is in such good shape that I thought I would see what the rest of you thought. It would be nice to see it work for another ten years.