The mystery is solved. My receiver is a simple stereo unit (HK 3470), so I pulled out the jumper pins from preout to main in. I replaced the pins with interconnect cable, but crossed the channels: left preout to right main in, and vice versa. This resulted in a right-channel bias, so the problem is actually the preamp section of my Harman Kardon.
I verified this on multiple inputs, including the tuner. This means that the Axioms are so sensitive that they revealed a slight L/R imbalance. I never got an inkling of this problem with my Paradigm Mini Monitors.
This same thing happened to me with my headphone amp: the Headroom Little. My Grado SR60 headphones are also very sensitive, and so at low volumes it sounded like the sound faded to one channel. I sent the amp back to Headroom and they kindly replaced the op-amps with a better matched pair.
By the way, my SR60s, which cost $70 new, are far superior to my Mini Monitors. This was one of the reference points that gave me the itch to upgrade speakers! While I haven't listened closely to my headphones much on my main system, I wouldn't be surprised if it's still a tossup between SR60's and the M22's. There's a powerful immediacy to good headphones, and I actually find the imaging very enveloping.
-Cooper
