[AkiTAKA]

so the sound on the left driver is permanently softer by quite a significant amount. this occurred after i tried to adapt it to a 1.5mm jack. the driver got very hot only on the left side and the driver got softer. would replacing the driver fix the problem? the headphone in question is the sony CD-900st studio monitor and replacement parts are rather easily available including the drivers.

 

[PETEREK]

Did you solder enameled wire or a different kind of wire? It could just be a cold solder joint causing that, or a bad connection to the ground wire for that driver. 

 

[nikongod]

  so the sound on the left driver is permanently softer by quite a significant amount. this occurred after i tried to adapt it to a 1.5mm jack. the driver got very hot only on the left side and the driver got softer. would replacing the driver fix the problem? the headphone in question is the sony CD-900st studio monitor and replacement parts are rather easily available including the drivers.

 
Probably need new drivers. Possibly dont. 
What does the multimeter say when you measure tip to sleeve, tip to ring, and ring to sleeve? 
 
Whatever you do, I would figure out why the driver got hot. 
 
Why not build an adapter to go from 1/4" to 1/8"?

 

[AkiTAKA]

i haven't done any modifications to it and neither have i ran diagnostics with a multimeter. I was just thinking if it was very likely that I would need to replace the driver which is probably the easiest part to replace. not too savvy with this sort of electrical stuff but the vented metal casing of the driver got quite literally scalding hot.

 

[PETEREK]

When soldering stuff like that, you need to be quick and precise. You don't want anything but the solder points to get hot.

 

[AkiTAKA]

the adapting seemed to have gone well. the driver started to heat up after i plugged the headphones in. sorry for my misleading english

 

[nikongod]

  the adapting seemed to have gone well. the driver started to heat up after i plugged the headphones in. sorry for my misleading english

No problem. 
 
The driver heating up AFTER you plugged the headphones in is a problem.
 
This can happen because the cable is wired wrong, OR because the amp is toast. In either case, it is not good.
 
Measure resistance everywhere mentioned above. Does everything make sense?
 
If all of the DC resistances make sense, measure the DC offset from your amp. Is it low? Ideally 0.0mV , but certainly less than 50mV. If it is more than about 0.2V (200mV) you should fix your amp. 
 
As a nasty curveball, if the DC offset of the amp is high - it is possible for the voicecoil to melt off of the "cone" without changing impedance! If DCR measures properly, but DCV did not and sound is messed up, this may have happened.  

 

[AkiTAKA]

assuming the amp is the problem and my headphones are well and truly permanently affected, what parts on my headphone should i replace?. i am using it fine now using channel balance on my PC but I can't use an external DAC or my DAP because the channel balancing software on my DAP is not adequate enough to correct the channel inbalance on my headphones.

 

[PETEREK]

It REALLY sounds like you're signal has a cold solder joint to me now. You're basically amplifying the signal of the one that is quieter to make it equal volume to the one that is soldered properly. Do yourself a favor and replace the wires you used on the drivers before with some new ones and resolder it. Don't buy anything new before you see if that's the only problem, it sounds like it probably is.

 

[AkiTAKA]

all of the wires are stock. i did not do any soldering in the drivers themselves