I personally would not do this. I think it’s just too dangerous for the amp, headphones, and your hearing of anything goes wrong. My buddy exploded a pair of Code6 headphones (LFF modded 6 screw HE6 headphones) off a Vidar, even though he was using a converter box that had resistors in parallel and series to step the power down. It also didn’t do great things to the Vidar because apparently such a contraption also wants to draw more power than the amp is designed for.
Edit: you would be much better off converting those headphones to run a balanced connection then you would be from trying to convert an amplifier to run single ended. Please consider doing this first.
See the post I made about why bridging balanced connections is bad, then multiply that bad by 100 for speaker amplifiers because you get that much more power from them.
If you still want to proceed…
IF (and only if) the speaker amp is only driven on the (+) terminal and the (-) terminal is ground, then bridging the L and R (-) terminals might not explode the amplifier (there are still no guarantees it won’t explode), and bridging these connections will have to happen to convert it to a single ended TRS jack. In other words, you’re still playing with fire, literally.
If you have a multimeter, you set it to measure AC and then can put the red probe on the (-) terminal, black probe on a ground connection (wall socket, ground point on the chassis, something…) and see if it’s driving anything when music is playing at really low volume. If it just sits at 0V, then only the (+) terminal is powered and you theoretically should be safe. If you see any voltage across that, do NOT bridge the L and R (-) terminals.
Now, you should also put protection in there. Many threads on various sites go over how to add a voltage divider to this connection so you don’t immediately blow things up. At bare minimum, a resistor across the outputs will help.
Reading:
https://www.head-fi.org/threads/speaker-amps-for-headphones.649107/post-9271604
Or, you can just get something which does all that in a single box. (It’s an XLR plug, I know.) Headphones & portable audio - HIFIMAN.com
But, this still won’t keep your headphones/amp from doing bad bad things if you accidentally bump the volume knob. RIP your hearing too.
