... from the what would you do department.....

I am currently using Senheiser 650 headphones which I had rewired for balanced operation.  I did this because all my electronics are Ayre which are fully differentially balanced.   I'm using a dedicated fully balanced headphone amp called a Headroom Blockhead which was Absolute Sound's top state of the art dedicated headphone amp when I bought it, but the company is now out of business.  The connections to the amp from the earphones are with 2 XLR plugs...the centers of the plug are open to accept the regular phone plug.  The sound of the 650's are beautifully musical....though the highs and lows are truncated....but the mids are excellent.....better than my Vandersteen 5A's.    Anyway, I just got
Sure SE846-CL phones.  When I plugged them into the accepting plugs of the amp, the sound was awful....mono I suspect, though each ear was working.  All the reviews say this headphone is very good...so what would you do.

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I'd get out a continuity meter and see what kind of jack's I'm dealing with.
Are there combo XLR/TRA like pro gear or what?
Are they XLR/TRS?
The inputs on the amp are called combo jacks......that is, there are 3 holes for the pins of the XLR and in the center of the 3 pins, there is a hole which accommodates a phone plug (TRS).  
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It’s hard to say exactly what the problem might be without more knowledge about the design of the amp and the special cable which apparently is used to connect to it, but I suspect that it is related to some combination of three things:

1) You’ve gone from 300 ohm headphones (which were "rewired for balanced operation") to 9 ohm headphones (which have not been "rewired for balanced operation").
2) The headphone amp is dual mono and balanced.
3) The ground part of the phone’s TRS plug (the "S" in "TRS") is common to both channels.

So you may be either shorting one of the two signals in the balanced signal pair that is being output on one channel to the corresponding signal on the other channel, or shorting signals from both channels to ground. Or both. And you may be doing it through a much lower impedance than previously. As I said, it’s hard to envision this precisely without having more knowledge of the design, but I suspect the explanation is along these lines.

The bottom line: I suspect it would be a good idea to return the SE846 if possible.

-- Al