That's what i was asking myself ... are ther only one main voltage in an amplifier or multiple, cause i would be able to remove the internal transformer for one external single voltage but not for multiple (seems too complicated).Amp designers need to control all aspects of the operation. If they allow third party power supplies or even DIY ones, then it could impact the sound quality of their beloved design, or worse, blow up, and they get sued!
If you want to risk a DIY job, you need to open your amp up to find out what voltages the amp is working with on the rails. It could be +/- 35V or higher.
Then jury-rig batteries to supply both voltages (the plus and minus). Run it on charged batteries.
Then jury-rig a charging solution to charge the batteries overnight! a bit of a hassle, you might realize, it ain't worth it, unless you really have to.
Now i got an old Yamaha RXV800, i will open it to see how it goes on the power input side.
The other thing is that i'm off grid .. and Class D amplifier are pretty energy efficient, that's an other point to take into account.