I would like to hear comments about whether anyone is hearing any background amp noise due to the efficiency of the PM-1. If the efficiency is really 102 dB, I have to think that noise would be audible on a lot of amps, especially older designs and tube amps. I notice that a lot of the newer amps have one or more gain settings, due to the fact that phone sensitivity can range from 83.5 to over 100 dB. I understand that Oppo wanted to tap into the market for the majority of people that will use a portable device, but I think the high efficiency will cause noise problems for a lot of Head-Fiers using separate amps. Since there is a 30-day return policy, at least you have the option to send them back if they don't work on your amp. An earlier asked poster asked this question, but I haven't seen any responses yet.
I'm using the HA1, but no audible hiss to my ears.