DO a search on Agon this topic has been recently covered and in the past.
From my personal experience, I think Elberoth2 has a point worth checking out. I paired a MF Tri Vista 21 DAC with my existing Arcam FMJ 23 CD player. With the amp I had at the time, I thought it was a great all around improvement. However,when I upgraded the amp to something with greater power and detail, I found a lot of high treble glare. At first, I thought it was the amp, or poor synergy with new speakers. Taking some advice,(quite counter-intuitive, might I add), I took the DAC out of the system; problem solved.
On the other hand, running the DAC with an old Theta Data Basic transport sounded great. The knowledgable retailer who advised me sells both separates and single box CD players, but is very leery of combining a DAC with a single box player.
Save some $ and get what I think is the best thing going right now:
the RAKK dac from Kevin Carter at K and K audio. K and K audio site
I had a Trivista 21 on hand to compare with, and the RAKK is better. (I have the passive version of RAKK with Tent Labs clock module.) It's not much more than the dAck unit (maybe less, depending on options), and easily far more capable in terms of performance and various options.
Filterless and minimalist dacs like dAck and shigaraki just don't have the quality of sound. Sure, they are simple but the price is paid in resolution and top end extension. It's just not there. (there is no free lunch. Yes, I have had some of these units in my own system for auditioning).
the RAKk on the otherhand is well thought out, comprehensive in its abilities, has many good options (tent labs clock module, active tube output stage, passive transformer-coupled output option, tube-regulated power supply option, etc.) and can also handle upsample/resampling and different frequencies/sampling rates.
anyone thinking about a DAC should look into the RAKK, IMHO. I have yet to compare it to my SCD1 but will do so soon.
You won't know your dac until you have a decent transport. I have a Scott Nixon TubeDac. Even though non-oversampling dacs are less susceptible to jitter, transport quality makes a very large difference. My system sounded very smooth and musical when I was using my Cary 303 as a transport, but somewhat brittle, unbalanced and boomy using a Tosh dvd player as a transport.