Seeking DAC advice

Hey All,

I'm looking to do an upgrade. My current system:

Windows 7 PC, WAVs and FLACs
E-MU 0404 USB 2.0 External
NAD C 316BEE Integrated Amp
Mint condition JBL 4425 Studio Monitors

I believe that the weakest link in the above is the 0404. Please feel free to correct that notion. Nonetheless, I was considering and upgrade to one of the following DACs:

Schiit Bifrost, possibly with Uber Analogue upgrade.
Cambridge Audio DacMagic Plus
NAD D1050

I listen to a variety of music, and value detail with as little distortion as possible.

My questions:

1. Most importantly, will one or more of the above be a significant upgrade over the 0404?
2. Is one of the above DACs the best upgrade option (in the $4-600 range) for my system?

Thanks in advance for your time!
Not sure if this helps, but when I bought my Wavelength Proton 4 years ago. Last weekend I did an A/B comparison to the $2800 Ayre QB9. The Ayre was better, but not by $2200. We when bought the Proton, we compared it to the Cambridge and the Proton was a no brainer. I'm probably going to upgrade soon to the W4SDac2se. You might check out the new Protons for $600 or let me know if you why to know more about mine.
Good luck.
Given that you have a integrated amp, and if you don't need the headphone option, I would not buy the DacMagic. One way to improve a budget DAC for computer (USB) use is to employ a USB/SPDIF converter. My approach has been to buy a older use DAC with known quality power supply and analog output stage and couple it with a USB converter. Have used a PS Audio digital link III with M2Tech Hiface converter. Since upgraded to Bryston and Audiophilleo.

A used Rega DAC could be had at your price point.
A Music Fidelity V-Link or Hiface and Non-USB Bit Frost could be had new also. With HiFace no USB cable is required.
The Parasound Z-DAC gets great reviews here and Stereophile.

Good luck in your search.
THe iFI iLink is another budget USB converter that is highly thought of.