Hi David,
Your first two threads on your upgrade dilema attracted many suggestions and advise. Which ones have you auditioned and what were your conclusions?
Regarding the MDAC as a pre amp, I agree with the above. I tried one in my system with a/b comparisons between the DACs in my CD and SBT - complicated a little by the various filter options available in the MDAC. I then removed my pre amp and fed my power amp direct. After 4 days of evaluation, I decided that the MDAC wasn't for me. It offers excellent transparancy and detail, but to my ears, is a bit clinical and lacking in emotion. I went through the same process with the irDAC and now forget the hifi and just enjoy the music. To my ears, it is superb and no fiddling with filters or unneccessary preamp function. Listening to compressed internet Radio Paradise via the Squeezebox has become much more enjoyable, even though the Dac in the squeezebox is suprising good for a budget product.
That said, as mentioned before, the price differential between a stand-alone dac plus cabling and the audiolab CD is not huge and might deliver better results than using your 6003 as a transport. I would check out Creek's new Evolution 50CD from the same stable as the audiolab. Moving up the price ladder, I was most impressed by the Marantz SA8005 which offeres the bonus of SACD and DSD streaming. It is built like a proverbial Centurian tank. Horses for courses and don't rely on reviews, IMHO.