I've been using PC350s for 5 years now and honestly they were never that good.
What sources are you going to be plugging the headphones into?
Headphones with an integrated mic are usually sold to gamers, who do not have or are unwilling to spend the amount of cash the HD600/HD650 cost.
Best headphones I can think of with built in mic
Beyerdynamic MMX300 (DT770 with an integrated mic), closed headphones.
Sennheiser PC360 (same driver as the HD518, HD558, HD598), semi-open headphones.
It's plugging into a Xonar STX.
I've tried the 360 and the 363D and they sound about the same as the PC350(The 350 actually sounds better with my amp) -- don't want to spend $300 on cans for little to no upgrade. The MMX300 are way too overpriced, they want an extra $200 for a mic? Money isn't an issue for me, but I don't like wasting it.
Been thinking about buying the 650's and doing that mic attachment to them, but they seem to be out of stock till rev4 comes out apparently.
I've been looking for months to replace these 350's, there just isn't anything that really fits the high-end audiophile headset that isn't a gimmick.
Guess you where comparing well broken in PC350s to a new PC360/363D, may not have been a fair test.
Sennheiser HD558s, used ones sometimes sell for around $100, you can remove the black rubber tape from inside the HD558's cups, should improve mids and treble, slightly less bass.
Needs hours of break in time to sound their best.
At 50-Ohms, really easy for Essence STX to drive.
Use any add-on mic you like.
You could try replacing the Essence STX's op-amps (operational amplifiers) might find some that improve the audio more to your liking, cheap op-amp upgrade, two LME49720, $3 each op-amp.
LME49990, $50 for 3,
AD797BR, $50 for 3