Would an external DAC improve the sound of my SONY

Hi! I have Sony NS 999ES SACD/CD and have been doing extensive A/B testing, trying to compare my Sony with Musical Fidelity A3 24 upsampling DAC, that I have on loan in my system for the last few days. Mind you, the MF DAC MSRP is the same, as Sony ($1200), and it's been highly acclaimed by Sam Tellig of Stereofile not so long ago.
My conclusion so far is: no matter how hard I tried to like MF DAC ( and I can get a real good price on it), I could only hear very marginal difference b/w the two. Possibly a little bit more refinement in the HF region, and maybe, just maybe, slightly better articulated mids.
But again, the difference is so insignificant, that I cannot justify paying for such marginal improvement yet. I really tried to like it.

Socrates! I'm glad (and sad at the same time) that you confirmed my impression of Sony 999 vs. MF3 24 DAC. What would you suggest instead for Redbook and Multi- channel SACD playback. I could actually do without the DVD at this time, will add separate dedicated DVD later on, as needed.
Actually, I ended-up buying the MF3 24 DAC. Appreciate you response in advance.
P.S. Read your review on Marantz SA- 14, great read. Our music tastes are pretty similar btw. ( Ian Anderson, Jethro Tull, Orff, Beck, DSOTM, etc.) Regards.

My question is now this: since I'm probably going to use Sony 999ES as a transport for Red Book playback with MF3 24 DAC, would doing the mods improve " garbage, coming out of the digital output" as was suggested by Socrates earlier in this thread. Or, should I forget about using Sony as transport, and just go ahead and modify it. And how one would compare MF3 24 DAC performance against, let's say Modwright first level mod? The price is not an issue, the Red Book performance is. I know, this horse has been beaten to death already, but still I want to know your opinions anyway. Regards.

Thank you, Socrates! Your point is very well taken.