Yamaha M2 channel mismatch

So my friend picked up a used Yamaha M2 for a decent price, and today I went to check it out and listen. This is the first time I'm seeing one in person and it's a monster! Insanely heavy and looks very industrial. I was surprised that the manual says this amp is rated to run only 8ohm speakers or above. What limits this amp's rating for low impedance speakers? The power supply looks well built .. I was hoping to find something similar since I am on the hunt for vintage infinity speakers, but maybe I have to look at M-xx models.

Before I rant on, here is the actual problem...

We noticed that the left and the right speakers don't have the same volume levels when the input attenuator in the back are set to the same values. We had to bump up the weaker channel by nearly one number (one channel at 1 and other at about 2) before the speakers sound matched to our ears. We systematically ruled out speakers, cables and preamp/sources as the cause. Both speaker A and speaker B outputs show this symptom, and the mismatch moves when the speaker wires are interchanged.

What could be causing this? My friend did not thoroughly test it before buying, but he says he did not discern a difference at the sellers place. Perhaps the volume levels were high enough. He has the option of returning, but if the fix is not too involved, we can try .. Both of us have some experience but not on a power amp.



(Sent from my mobile, please pardon my spelling)

Another question - are we supposed to keep the input attenuators on the amp all the way up (ie no attenuation), and adjust volume on the preamp to what is needed, or the other way round? Sorry if this sounds stupid, neither of us have used a power amp that had level control pots.

(Sent from my mobile, please pardon my spelling)

If you are having a slight problem with channel mismatch, you should thoroughly clean and DeOxit all the input and output connectors, speaker fuses and fuseholders, attenuators or potentiometer / volume controls, and any switching (like speaker selectors) if applicable. If there is a speaker protection relay, its important to check the relay contacts, renew or clean if possible, or replace the relay. All of these contact points can get of build up corrosion and that can incrementally decrease the sound. Sometime exercising the contacts or frequent sound on - sound off cycles can burn off some of the corrosion, especially if the amp had been previously sitting for awhile.

Then download the service manuals, and make sure offset and bias are correctly set.

Personally, if the slight difference is less than 1-2 attenuator clicks, and otherwise seems ok, I'd just set them where the balance seems the best and forget about it.

As for attenuation settings, if you wish to limit ultimate power to speakers, you should turn the settings down or back to around 60-75% and then use preamp. If you want best signal to noise, have them around 80-95% of max. If you want all the power available, full on.