I think that something that some folks are forgetting is the ANALOG in D(igital) A(nalog) C(onverter).
The biggest difference to be heard from most DAC's is IMHO due to the presentation of the now-converted digital signal by the analog stage. I've been told by folks who know that this is what makes the biggest difference in the quality of sound...there are a relatively few DAC chips, and the benefits of better analog staging and power supply have been well documented. THIS is what you get from a better DAC (although the technology used to convert the digital signal may be superior from one DAC to another).
BTW - I've used MSB and MF DACs for coming on 10 years now, and with excellent results. Jitter can be an issue, but there are products available to deal with this (see Monarchy Audio and G.W. Labs)...and some manufacturers have proprietary technology to deal with it rather nicely (e.g. MSB Network).
The long and short of this is that most folks can get a nice benefit by using an external DAC...not so much by improving the DIGITAL signal (though that's possible), but by improving the ANALOG stage.