[aeberbach]

I have PSB M4U1 - they are very good! But I'd like better, I have tried one better option already. It must be closed (office use). I prefer neutral/accurate to bass-heavy. If an amp is required, can do. Budget is $1000+ - so what do you think is the best available for a wide range of music?

 

[kapanak]

Unfortunately, what you are looking for is a ... unicorn. No such thing as state of art closed back headphones that offers isolation and doesn't leak sound AND sounds like a $1000
 
Some here will offer the Alpha Prime by MrSpeakers as an option. But that's a mod, a severe mod that makes it seem like a whole new product, but in essence still a mod, and it has severe variation problems in the production line, not something to take chances with using big money. Just don't go this path.
 
There is an unproven Audeze EL-8 closed back, which you must await reviews and impressions for.
 
Fostex TH600 and TH900 offer really minimal isolation, and leak a lot of sound, so they act more like open back than closed back headphones.
 
LCD-XC is probably a very good bet, although if you are sensitive to treble spikes, I'd advise against it. It is also 1.5 pounds, so quite heavy (more than twice your current closed back). 
 
An interesting option is LFF's Enigma, and the less known Enigmatic headphones, which are entirely handmade by one person (Luis) and are custom tailored to each person, and use custom drivers (they are not mods of other existing headphones). If I wanted to buy a closed back in the $700-$1200 range today, that actually worked like a closed-back headphones, these two is what I'd look for.

 

[aeberbach]

You don't rate Beyerdynamic T5p at all?
 
I have read a little about the Fostex TH600 - opinions on leakage are very mixed. 

 

[kapanak]

  You don't rate Beyerdynamic T5p at all?
 
I have read a little about the Fostex TH600 - opinions on leakage are very mixed. 

T5p is a total mess. Bright and sibilant, and has distortion up the wazoo.

 

[semirandom]

Been searching for a vaguely end-game set of closed cans myself recently, and... I'm reluctantly coming to the conclusion that there is very little (sonic) improvement to be had beyond the $300-500USD range.  I will say that if you have some wiggle room in isolation and leaking requirements, the Fostex TH-900/600 could serve as a sort of best-isolating/least-leaking *open* headphone family, but anyone calling them closed is being a bit silly.
 
As for the T5p... I found their sound signature intriguing - I can even enjoy profiles that others find distasteful (a unabashed modern Grado fan, for example), but I couldn't get over what seemed to be some damping issues.  Every once in a while, I'd get this unpleasant resonance that I thought was going to shake my teeth out or something.
 
Personally, I'm on the verge of abandoning my search and plowing my accumulated closed can budget into better IEMs, but best of luck to you.