I just could not believe Zeos that my beloved THX 789 + RME ADI-2 DAC combo should not be able to properly drive the DT880 600 Ohm. It is correct that I don’t like the 880 on this amp in SE (quite anemic) but I am very happy with the THX driving balanced e.g. my old Beyerdynamic T1(1st Gen) 600 Ohm (I modded that to balanced as well) or the relatively hard to drive AKG K701 (also modded) and many more headphones I own (HD6xx/HD660s/ DT1990/ Neumann NDH20/ T5p2/ Sundara/ SendyAiva/ Helios/ M1060s/etc.).
Nevertheless, the main idea of this mod is not to add fancy connectors for detachable cables to the cups but to have a quick mod and to simply use what is already available and not to alter the cups at all - so that you may test and undo this mod without any problem. You will have to unsolder 5 connection points and to solder 2 - but that is simple and can be easily undone. You only need 1 wire which matches the wire from the left to the right cup in length/strength (the lazy approach is to get one replacement wire from Beyerdynamic for 5 bucks).
The drawbacks are that a) you have to flip both cups upside down - but they are completely symmetrical - hence it is only a cosmetic flaw with the text on the labels (and you will probably not go outside with it anyway). and b) the wire fixation is not superstrong at the cups (not to be used for sports).
Of course you yourself are responsible for any damage you make to any gear. This is not a recommendation but my experience report on what I did.
You only have to open the left cup with the cable and keep the right one untouched.
Unsolder all connectors from the driver (remember where they were )
Detach the clamps that fix the cables (main and inter-cup) and remove the stock cable. Pull the wire to the right cup through the headband (you may also open it but it is not needed). Put a tape on the hole from the stock cable (on both sides with some foam inbetween to isolate better).
solder the new wire to the driver (measure resistance/keep polarity in mind) and put it through the inter-cup-connection hole.
Unscrew the cup-holders. Flip the cups. Now there is less freedom of cup movement since the newer generation Beyers have an orientation in the small plastic stoppers on the cup. If you need more flexibility, cut a part of plastic from the holders (then the “reversibility” is obviously broken, but you may buy for 5 buck spare parts from Beyer in case you want to undo it).
Connect and fixate the two wires to the 4 pins of your end connector of choice (4pin XLR in my case). Done.
Obviously you could have simply attached a 4 pol balanced cable inside the left cup to the driver connectors and to the inter-cup-cable and keep the rest as it is without any cup-flipping. But
a) I do not like to cram stuff into the cups (I guess the original sound designers/engineers wouldn’t have liked that - and now there is even less difference between left and right cup “filling”) and
b) I dislike cables going from cup to cup and prefer the cables to be on both sides (physical balance). One additional advantage is that the wires do not point downwards to your shoulders but forwards.
You now own the 088TD balanced headphones Sounds great on the THX 789 balanced output !