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.

Medium to high-cost Sonys make great transports once they are modded properly, particularly the DVP-S7700. My personal DAC favorites are modded Perpetual P-3A and modded Birdland Odeon-lite.

Thanks for the response guys. I think I will keep the changer "as is" and look into a transport/DAC combo for more critical listening. If you have any ideas fire away
Bob.

The Sony 999 dvd/sacdp is a HORRID transport, lots of garbage comes out of the digital output, that's why you hear little difference with the MF dac, which sounds stunning in the proper system. Get a proper transport, even a cheap Theta Basic or PS Audio Lambda connected to the MF dac and it will eat the Sony alive on redbook. The Sony 9000ES is a much better transport then the 999, if you must use a dvdp as a transport.

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.