The reason you hear a bit of hiss with the HD-650 and not with the Maddog is that the HD650 is a higher efficiency/sensitivity headphone. The models higher up in the Pioneer line have a -20dB muting feature which both reduces/eliminates that hiss and also allows more volume control play.
I always wondered what that -20db switch was for. My Onkyo 6500 mkii has that button and Ive never used it. I certainly have a number of albums and cds that have quite a bit of hiss. A remaster of DSOTM has a ton of it, both on headphones and HPM-100s, Ill have to give that little button a push and see what happens.