Well, I stubbornly bought the HPA2s since they are half the price of the Sennheiser PC350 headset. They just arrived today.
Unexpected feature: they double as a wifi signal detector. If the amplifier module is anywhere near my wireless router, I can literally hear the wireless signal. It sounds like a repetitive tapping, but when I put the amp right on top of the router, it becomes very loud. If I enable wifi on my phone and browse the internet, the tapping becomes a muddy beat. Still not convinced, I disabled wifi in the router, and the sound went away completely. Unfortunately, I can hear the wireless signal with the amplifier module pretty much anywhere on my desk. I never pass up an opportunity to make lemonade out of lemons, so I mapped out signal strength within a certain radius in my living room thanks to the huge cord.
I can also hear my mouse movements through it, they come across as a tiny squeal.
These might be going back to Newegg...