What difference would the XYZ DAC make?

I've spent some time searching and reading past Audiogon, Audio Asylum and Audio Review threads on DACs. While the comments are informative, they tend to fall into three general groups -- the "I own the [XYZ] DAC and it sounds great in my system so you should get one" self-validation group, the "[XYZ] DAC is detailed and images well" technojargon group, or the "Don't waste your time with the [XYZ] DAC, I have an [ABC] DAC and it just flat blows the doors off of that outdated old [XYZ]" in-your-face group. All are at least somewhat helpful, but... I listen mostly to small jazz groups featuring guitarists who focus on subtlety rather than slam (lots of Jim Hall, Wes Montgomery, Pat Metheny and Joe Pass), although I also listen to some classical, rock and folk. My system is nothing fancy -- a Cambridge D500 CD player (the original one, not the SE model), playing through a NAD C340 preamp section, a NAD C270 power amp, and Vandersteen 1b speakers, connected with MAS ICs and Signal speaker cables, with a Cardas power cord for the CDP. I tend to go for a warm and musical sound more than a clinically neutral or hyper-detailed sound, and value natural musicality above other characteristics. I do also look for clarity, detail, imaging, bass and treble extension, and pace, rhythm and timing, though. These are all things that I think I can recognize if and when I hear them, because I've been playing and listening to music for a long time. A used DAC seems a realistic place to start in taking this system another step further. We're talking units that would be cost-proportional to the other equipment:
-- original MSB Link DAC, unmodified
-- Link DAC III with Modwright mods
-- Cal Labs Sigma II tube DAC
-- Channel Islands Audio VDA-1 DAC
-- McCormack DAC-1
-- Theta Pro Prime or Basic DAC
-- Van Alstine Omega III, Omega IV or OmegaStar DAC
No helpful suggestions to try a Wadia or the latest Cary, please. I'd probably like the sound, but it wouldn't make a lot of sense in the context of my system and budget. No sneers warranted either, since my two kids in college are both higher priorities than my audio toys. Just straightforward answers (or scintillating displays of erudition) in response to two simple questions:
(1) Based on your personal experience, in what specific way is the sound of my current system likely to change with one of these DACs?
(2) Which of these DACs, or others available used in the same general price range, do you consider most likely to improve the sound of my system in ways consistent with my tastes in music and sonic characteristics, and why?

I'm in the same boat - looking to upgrade my modest high end system. I'll be interested in reading responses.
I've been researching possible upgrades, ran a thread similar to yours and got many good suggestions. I decided to upgrade through room treatments (doing it now)and then a DAC(researching used DACs now). I'm currently leaning toward the Musical Fidelity Tri-Vista 21 DAC - used here for around $1400. A little expensive but it seems to fit what we both like: warm and musical but also detailed with good imaging and presence.

Good luck in your search.
I many ways your tastes in music are similar to mine: as you said, "focus on subtlety rather than slam". In terms of your questions, all I can offer is what I have learned in my own experience. My suggestions would include another candidate; the Audio Logic M34. It may be a bit more than your budgeted list, but is a very worthy candidate. It is a tube DAC, and is very sensitive to tube selection (particulary the 12AU7s) which gives you a lot of latitude to tailor the sound to your own preferences. I own this piece and have no thoughts of parting with it. Perhaps Pat (Rx8man) will chime in on this thread....he had one of the tubed Cal DACs and now has the AL34. He could clarify his preferences (I know he is happy with the AL).

The downside of the AL DAC is that it is thinly traded in the used market. Yes, you may be able to read something into that.

Two last things I would offer from my experience; I did not really begin to appreciate the AL DAC until I put a good transport in front of it (for budget purposes, there are good transports available in the $500 price range), and secondly, both the transport and the DAC are (IMO) very sensitive to loving good isolation.
For a system like yours, I'd skip a separate DAC and try one or two standalone players you think may make for an upgrade instead.

One reason for this answer is that yours seems like a simple system and so I assume it should probably be kept that way, i.e., if you go with a separate DAC you could be looking at needing another power cord (and maybe outlet), a digital cable, and maybe another rack shelf.

Another is that transport quality is critical to overall success in this area, meaning if you're like me, before you're done you'll have gone to a dedicated transport over your old player, and quite possibly an outboard jitter-reduction box (and yet another digital cable, PC, outlet, and shelf) as well.

As for choosing what to get, IMO one of the beauties of Audiogon is that it enables one to easily experiment with used gear and not be forced to take a big loss if something doesn't work out. Bring in a couple of likelies, and sell what doesn't stick.

About my own experience with items similar to what's on your list, I've owned both a Theta DSPro Basic IIIa and an MSB Gold Link/P1000 Power Base (both the top iterations of their respective models), and greatly prefer the Theta, for reasons including the ones you list when you say, "I do also look for clarity, detail, imaging, bass and treble extension, and pace, rhythm and timing", in addition to tonal neutrality (but with full-throated color saturation), dynamic authority, dimensionality, grainlessness, and soundstage spaciousness. The Gold Link is 'warmer' FWIW, but inaccurately so, though some might find the Theta "clinically neutral or hyper-detailed" I suppose, just because it doesn't color or glaze over anything, including nasty-sounding disks.
Having spent time in both camps, I would point out that Zaikesman has given very good advice. His words regarding the importance of transport selection is in my experience especially true.