Need a DAC recommendation

I would like argue the importance of the overall design of a DAC, such implementation of the output stage (opamps, tube, discrete fets)and power the supply, which some say is more important than the dac chips and bit rates.

However, taking this 180 degrees, I'm blown away by the performance of my $150 HRT Music Streamer II (after 300 hour burn-in).... which uses cheap opamps but asynchronous USB technology... I think that the $350 HRT II+ should be on your shortlist if USB is all you need.

Agreed, the importance of the Analog circuit and quality of components is crucial. But in the terms of value, digital has come a long way when we look at sub $600 category (i.e. this thread), and the $150 HRT Streamer in my office system smokes my old Cal Audio DX-1 (a $600 player in 1994)in terms of resolution and soundstage. I think, too, that the current technology does a lot better job addressing jitter.

If you will only be doing computer audio, then I urge you to research USB DACs. There has been a lot of development recently, and USB now handles high resolution files. There are two approaches, asynchronous (e.g.,Ayre, Wavelength, HRT, Wyred 4 sound) and high resolution synchronous (CEntrance, Benchmark). Within your budget, I recommend you'd look at HRT Music Streamer II+ (the II+ will have much better analog output stage and components than the II) or the II Pro if you have XLR inputs on your preamp. The CEentrance products should also warrant your consideration.
Is the high resolution USB as good as the digital coax? Depends on a lot of variables. How are you getting coax out of your computer? Musical Fidelity V-link or M2Tech hiFace Asynchronous USB To S/PDIF Converters? Or just a basic computer soundcard?
I am assuming the latter. Why don't you borrow the Musical Fidelity combo from your other system, and hook the V-DAC up both via coax and through the V-Link, and listen which connection sounds better. That should give you an idea what direction you should go, coax or USB

I think you have a great combo with the rainmakers and the C375BEE to start with!
A lot is going to depend on the quality of the digital out of your soundcard- i.e., what route to take. The V-DAC appears to be one of those rare over achievers at its pricepoint- except for USB input. I don't think the Cambridge DacMacic or the Emotiva will make any more real music.
The PS Audio Digital Link-III recommended above has a discrete output stage (no opamps), 192k capability, and on the used marked, is a great value. I'm positive it will better the V-dac. However, its USB input is old school limited to 48/44.1. It would be my recommendation based on your original post/question (coax digital in), but I suspect that the V-link will outperform your coax soundcard-output by a huge margin.
You raise a good point about the power supply to the HRT streamers... I don't know how they do it... but they claim to isolate the power from the audio - and it sure sounds good. Don't overlook the CEentrance Dacport XE, it may even be better than the HRT.
In my main system I have Cullen modified (stage IV) PS Audio D-Link-III fed by Squeezebox Duet. The sound is extremely airy, neutral detailed, but still warm with strong bass. I have never heard an un-modified unit, but I suspect similar virtues would be there too. You could pick one up used for about $450-$500, and add a good USB to Coax converter that does 24/94- or even 192- and be just a tad over your posted price tag.... or do a HRT Streamer II+ and be done with it....