Cheap DAC for music server and cheap CDP?

I understand this question has been asked numerous times before in this forum, but I'd like some aditional input on the following two questions:

1.) Is there any DAC in the sub $500 that would considerably improve the S/PDIF stream out of my Digidesign mBox?

2.) At this price point, is there going to be any major sonic difference made to a cheap DVD-A/SACD transport?

Thanks in advance...
I bought a Cal Labs Signa II Tube DAC on EBAY for about $200. It’s hooked up to a SPDIF out on my computer. I would say it's a significant improvement over going straight into an AV processor. I imagine a DAC like a Benchmark, etc.. might sound even better but for now I'm pretty happy. I'm not sure what a Design mBox is though. My SPDIF comes straight out of my computer processor.

I'll add that another significant improvement happened when I started using Exact Audio Copy (EAC) to rip my CD's. There's a review at 6moons if you’re interested.
There could very well be a good improovement, but your ears can only figure that out.