Best DAC $200 and under but possibly up to $300

Hi everyone, I've looked up the past 10 pages and am still trying to find more info. I'm on a much more limited budget than most people here, otherwise I'd get a Benchmark DAC1, iRoc, etc. I'd really prefer to spend $200 or less, but I'd be willing to go up to $300 if it really made that much of a difference in light of my equipment. My computer (Mac) will be my source and it only as a USB or optical digital output. I'll be using a Rotel RA-1062 with B&W CM1's.

The one's I've come across but aren't sure what to make of them:

E-mu 0404 USB
Beresford TC-7510
iBasso D2 Boa
Zero DAC
Adcom GDA 600 or 700

Some of them are more DAC/amps for headphones, but I heard they were good. I'm just looking for a no frills DAC. Used, New, just whatever provides the best (or a very good) value.

If anyone would like to comment on the ones I've listed, offer their own suggestions, or whatever, I'm OPEN TO ANYTHING. Any info is greatly appreciated :D My head is just a little dizzy from all the spinning of all the stuff that's out there.
Blue Circle USB Thingee, $169.
I actually did consider the Blue Circle Thingee (as ugly as it is), but I don't want to use USB powered DAC's because I think it'll compromise the sound. It would be so much easier if I could go up a price level as there seems to be more uniform opinions on great DAC's (Benchmark, Bel Canto, Musical Fidelity, Channel Islands, etc.). Considering the $200 price level, there might not even be any clear 2-3 winners. I am leaning though toward the E-mu or the Adcom, but I'm not sure.

On a side note, does anyone know how much one should technically spend percentage-wise on each piece of equipment? Speakers: Amp: DAC? I'm kind of curious about that.

A correctly engineered USB DAC is superior to SPDIF imho.

I guess I should be more specific - I meant that I didn't want it to be powered THROUGH the USB and would like something that is plugged into the wall.