is it possible to get more wattage out of this with a higher volt power supply?
The power amplifier chip Merus MA12070 has a maximum voltage rating of 27.5V, so there is not much headroom there.
If you feed 8 Ohm speakers, the power supply voltage and amperage are actually pretty well matched: maximum current with 24V into 8Ohm is 3A, so for two channels 6A, which is not far from the 6.75A the power supply can deliver. You get theoretically up to 24V*24V/8Ohm=72W per speaker, but the Merus MA12070 chip specs say peak power at 8Ohm is 45W (10% distortion), 35W at 1% THD, and 30W continuous without heatsink (all per channel).
For 4Ohm speakers, you could use a higher current supply, since you could get 144W per speaker (assuming your speakers can handle that), but the supply maxes out at (less than, due to amp efficiency and use for DAC, preamp etc.) 162W.
shows a teardown of the A30; the good news is the big caps are 35V rated (Nichicon 2200 microFarad), but the bad news is there is a bunch of smaller 22 microFarad 25V caps:
The author of the video doesn't describe the components, but the screenshot above does show an XL1509 DC to DC converter (top left), and both the DAC chip and the volume control chip do definitely not use 24V as their supply voltage. But those 25V caps, assuming they are attached to the power amplifier chip supply, are kind of setting a limit; also, the 85℃ rating of the Nichicons means you don't want to run this thing too hot. It's nice though that the amplifier chip is heatsunk against the case.
TL;DR: amplifier chip maxes out at 27.5V; the power supply is well matched for 8Ohm speakers, if you run 4Ohm speakers you could try to find a 24V 12A (or more) supply. Increasing the voltage will most likely make some 25V rated caps explode rather sooner than later.