Loud Hum with Fubar DAC & NHT PVC

For a job recently I had to go mobile and leave behind my Musical Fidelity A3.2 Amp and Totem Hawks. I brought my W4S DAC-2 along and picked up a pair of KRK Rokit 5 powered monitors. This combo sounded great.

When I got back, I decided to disjoin my home system from my work system. The 1/8" jack out of my Mac Pro is terrible. I started looking into an affordable DAC that has a headphone amp and line output to my Rokits. The Fubar IV Plus sounded like a good deal so I picked one up. I was really happy with this at first. It sounded great. But an issue became hard to deal with. The line output from the DAC isn't adjustable. There's no volume pot. And it's a pain to adjust the volume for each app. When I'm video editing, I don't lock in my volume until I'm mixing the sound.

My solution was to order an NHT PVC passive controller. I hooked it up. Suddenly there's all this loud humming and feedback in the sound. Throughout the whole attenuation range there are one or two spots when the hum is minimized, but overall it's horrible.

None of you will be surprised that the hum in the Fubar set up went away only when I unplugged the DC wall wart to the Fubar.

I tested the PVC by putting it between my W4S DAC and the Musical Fidelity amp. The hum went away.

It looks like Firestone Audio does have an external power supply, but it looks to have different connectors than the simple DC power connector on the Fubar and I've only seen it for European power standard.

I'm looking for either a solution for this set-up, i.e. a power supply that won't hum. Or maybe another DAC that's within my budget that has optical input, a headphone amp and an adjustable line out output. THe Fubar and NHT cost about $279 shipped.

There's my long sob story. Any help out there?
LOL watch for the brand names especially FUBAR :-)
Marakanetz, We didn't listen when they told us to watch out the the speaker makers' haircuts.