He's right about the 10uf cap connected to the 100k. That will be about 3db down at around 60hz. I wouldn't do this personally, since I am a bass player, but for guitar, your lowest fundamental is 82hz so it's a good idea.
I'm not a power supply whiz, but it seems unlikely to me that 60hz hum would be coming from the power supply, since the supply is a SMPS. Hum is obviously an issue with linear supplies, but I thought, but could be wrong, that in a switching supply, the power is going to be heavily filtered and chopped to the point that the mains frequency is completely gone. Hopefully one of the more learned gurus will chime in.
Did you ever try a 3-prong supply? I am using a grounded supply and I have never had any hum, even with the first perf board prototypes that weren't in metal enclosures.