The P4 does a pretty good job with the HD600 when I compare it to my LD MKIII. The biggest difference seems to be the bass impact/extension and soundstage. The P4 seems to be more upfront sounding with less depth so it's a little flatter and not as full sounding. I also use the Q701 with the P4 and it sounds good but i don't have anything besides the MKIII to compare to so i cant comment too much. I don't know how useful this is because of all the different opamp and tube variations that come into play here but hopefully it helps some.