I think Ian is on the correct path. I have some questions:
1) When you connected the Axioms directly to the H/K, was the H/K configured to be both a Pre and Power amp?
2)If you have the HK (preamp) connected to the Parasound(amp) with the Axioms(speakers) attached to the Parasouns(amp) - you mention the buzz is there even with the preamp volume turned down?
3) are you connecting both old and new speakers with the same speaker cable?
I suspect that you have a problem with a ground. try this:
- leave the amp connected to the Axioms,
- disconnect the interconnects from the preamp, at the amp input, BOTH CHANNELS!
- check if the buzz is present.
This test should have No preamp, No interconnects, No source = only amp, speaker cables, speakers
What I suspect is a ground potential problem between the amp and preamp. As Ian mentions, an interconnect can cause this problem by "electrically connecting" the pre to amp via a cable conductor or shield. It would only take one interconnect in this situation to cause a ground loop that would be present on both channels.
If the amp and speakers do not make a noise, that is the problem.
If the amp and speakers DO make the noise (with no preamp, or source - amp to speakers only) I believe your amp could be the problem.