Your experience makes perfect sense. As you may recall, I identified to Gary Gibilisco, the designer of the iamp, the problem with the infrasonic response. John Dong offered me the same solution which I knew would not work. My iamp200 was modified by Gary to better filter out infrasonics. I now also use fdecks's HPF/Pre (which I helped a bit to design and test). You are 100% correct that the HPF cleans up all the clutter while actually improving the sound. The reason is as I just posted above. If you apply John Dong's solution, you end up cutting a band of frequencies centered at 40 Hz (the lowest setting on the frequency slider). By the time you affect the infrasonics enough (and it's not likely you really can), you'll pretty much destroy the tonal balance of your sound. To clean up infrasonics and judiciously cut the low end, you really need a HPF with a transfer function as shown here.