Each to his own opinion I guess .. I am wondering whether the fact that you have more POWER now using two amps vs. one amp and you now have more about 3 dB more headroom, could be a factor in your hearing better sound using the amps in the mono mode?
I have always found that vertical biamping your speakers using separate channels on an ST-70 type amp sounds best to me. In a case like this you would remove the straps between the woofer and tweeter sections of your speakers (provided they have separate inputs for both woofer and tweeter sections). You then run one channel of an ST-70 on the woofer section and the other ST-70 channel of the same amp to the tweeter section. You then do the same thing with your other ST-70 on the other stereo channel.
When you run an ST-70 amp in mono with 16 ohm speaker taps strapped, the feedback lines for the two channels are combined and this usually results in the sound quality going down. If, however, the mono mode works for you in your music system, then keep using it ..