2ch to 1ch... Stereo to Mono issues.

Once a long time ago I visited one of friends who had Genesis 5 speakers and Sonic Frontiers Power3 amps. One of his amps broke down and was sent for service to Canada. He was feeding his mono-amp from both channels through Y-connector. It seemed that 160W/side wasn't enough to drive even speaker with active(400W?)bass driven system and I'm sure it wasn't his best day even if he wished to demo his speakers. He wondered why from one (only right or left) channel the music comes louder than from both if brought onto one speaker and from two channels might even clip but not fill the room with enough sound.

I recently was running out of two channels preamp out through y-connector to the mono amplifier(certainly I'd prefere a complete music rather than listening from each channel separately). I was facing absolutely the same problem or maybe fact and wondered why I don't have enough gain or enough power and listened almost to the end of preamp's volume controll... Than I tried to feed my mono amp only from one channel right or left and had to turn the volume significantly lower than previousely. Than I played mono record feeding a mono amp from both preamp channels with no compromise to the gain.

I guess that out of phase signal cancellation takes a vast place in transfer of both channels onto one.

QUESTIONS:

What is the best way to bring stereo signal onto mono? Are there more reasons why the signal decreased becides the one I guessed?

Please note that in both cases amps and preamps were unbalanced. Preamps were dual-mono with separate ground for each channel but with one power supply for both.