I don't know about "best", but I've been using a McCormack TLC-1 Deluxe line stage preamp for about 4 months and have been very happy with it; it's quiet and transparent (what more do you want from a preamp?). I also just added an Aragon D2A2 outboard D/A converter to my system and couldn't be more pleased. Aragon is well known for their amps, but their D/A converter is a well-hidden secret. The D2A2 is very smooth and liquid, it opened up the midrange on my system, smoothed out the high end, and brought the low end into better balance with the rest of the musical picture. It's brought great listening pleasure and I think it's worth a look. It's $999 list new, I picked one up used for $525.
The best used preamp and d/a converter
My system consists of Revel's F-30 performa's and dual classe 100 amps, and a old Rotel Rsp960ax preamp and a denon cd player. I am definetly limited by the last two on that list and want to upgrade these components. I have been told the cp-50 or cp-60 would be a great preamp for my system and the theta IIIa converter with the data basic II transport would make a very nice system. I was just wondering what everyone else's thoughts are on this. Other recommenadtions would be helpfull. I am looking to spend about 1600 on the preamp, 900 d/a, and 450 on the transport. Any info would be appreciated. Thanks