Closed cans similar to HD600, Isolation,comfort concern

I am a very happy HD600 owner except when I am working and people around me are talking.  The sound is fantastic, but so is the clarity of the conversations.  I also have a pair of ER4S, but am looking for something a little more comfortable for some non-critical background music while im working.  I primarily listen to classical and post rock (explosions in the sky, mogwai, etc) while working, but also listen to a fair amount of rock when I need a little more of a distraction. 

I am mainly concerned with clarity and accurate reproduction, but being as these are going to be used primarily in non-optimmized listening environments and for background music for extended periods of time, I am more concerened about isolation and comfort at a slight hit to audio quality.  I currently do not have a portable headphone amp but am willing to purchase a Fiio E11 if my laptop and iPhone need a little help.  In my desktop I have an Asus Xonar STX so I have plenty of power there. 

Price range is a maximum of $400 (including amp if needed).  I am willing to buy used. 

I have read good things about the Audio-Technica ATH-A900X in terms of sound quality without breaking the bank, but have also not seen anyone citing them as having great isolation.  Alternatively, the Ultrasone PRO750 should have great isolation and clarity, but I have read that they are very harsh.

Obviously I am aware that isolation and sound quality usually are competing factors. 

Any other suggestions?

Thanks in advance!

I had the same problem that you're having now a few months ago. I have the 650's.

My requirements went like this.
1. They must sound good. Preferably they should be relatively neutral sounding. 

2. They must be comfortable. If they aren't I wont wear them. 

3. They must isolate both ways. I don't want to hear other people and I don't want them to hear me.

The headphones that I found after a few days of searching were the AKG K550's and I must say they have been wonderful. They're extremely comfortable. They Isolate to a surprising degree and their sound is good. 

I actually prefer my K550's to my HD650's if there is no amp involved. As soon as I put the 650's on an amp they win without contest. The 650's don't just need an amp for the extra volume...  they lose all of their life even at lower volume levels without one.  

Other headphones you might consider are:
Audio Technica ATH-M50. This one is good. Comfortable, Isolating, but it has a slightly tinny quality compared to the 550's and well they might at half the price. 

VModa M-100. This is definitely a bassier option and it lacks some of the detail that the K550's have but its easy to drive and is comfortable. This pair of headphones holds tighter to your head than a K550 which has a light touch. I find the K550's to be slightly more comfortable but its down to preference here.

Fostex T50-RP (With Mad Dog Mod) I haven't heard these but they intrigue me and they might fit your requirements. 

Consider getting a Head Streamer. It will make a night and day difference with whatever you buy.

I am a very happy HD600 owner except when I am working and people around me are talking.  The sound is fantastic, but so is the clarity of the conversations.  I also have a pair of ER4S, but am looking for something a little more comfortable for some non-critical background music while im working.  I primarily listen to classical and post rock (explosions in the sky, mogwai, etc) while working, but also listen to a fair amount of rock when I need a little more of a distraction. 

I am mainly concerned with clarity and accurate reproduction, but being as these are going to be used primarily in non-optimmized listening environments and for background music for extended periods of time, I am more concerened about isolation and comfort at a slight hit to audio quality.  I currently do not have a portable headphone amp but am willing to purchase a Fiio E11 if my laptop and iPhone need a little help.  In my desktop I have an Asus Xonar STX so I have plenty of power there. 

Price range is a maximum of $400 (including amp if needed).  I am willing to buy used. 

I have read good things about the Audio-Technica ATH-A900X in terms of sound quality without breaking the bank, but have also not seen anyone citing them as having great isolation.  Alternatively, the Ultrasone PRO750 should have great isolation and clarity, but I have read that they are very harsh.

Obviously I am aware that isolation and sound quality usually are competing factors. 

Any other suggestions?

Thanks in advance!