however there was a significant hum - whether its from the ripple of the DC, interference somewhere, or the lower voltage, I'm not sure yet!
If it's 50/60Hz power line hum it's probably not from the power supply because computer power supplies are usually high-frequency switching power supplies.* And, if it's designed to run-off AC, it should be better/quieter with "double filtered" DC.
The thing will probably "run" from a pair of 9V batteries. If you get hum when operating with batteries you know it's not power supply hum.
Mic preamps are high-gain and they can be prone to hum pickup. If the board is picking-up "stray" power line hum, the hum should change as you move it around, re-orient it, or put you hand next to it, etc. Connecting a mic might lower the hum because of the microphone's low impedance. But on the other hand, the mic itself, or cable, can pick-up hum.
planning on using a dynamic mic
If it has an XLR connector, that's a [u]balanced connection[/u] and you'll get better results with a proper microphone preamp. The balanced low-impedance connection cancels hum-pickup from the mic and cable.
The cheapest way to get a proper microphone preamp is to get a [u]small mixer[/u] that has a mic input. (You don't have to mix with it.)
- I had some little computer speakers with a missing wall-wart power supply once. So, I had the "bright idea" of tapping-into the computer's power supply and I mounted a power-outlet jack in a blank PCB bracket on the back of the computer.... The noise was terrible and I had to just buy a power supply.