BWentler,
I have the Pioneer 47Ai modded by Dan Wright, which includes the tube mod for the L/R channels. It's a serious improvement over the stock 47Ai. Since the 45A audio section is supposed to be identical to the 47Ai, Dan's mods should result in the same audio improvements. I will eventually post a review that compares this to my other players.
That said, I have listened to a stock 2900 in my system. Gobs of resolution / detail, but I thought there was a certain amount of cohesiveness missing. However, I would guess that a modded 2900 (by Dan or others) would outshine my modded 47Ai from an audio perspective.
How important is the video to you (and relative to audio)? If you wanted to upgrade your DVD player down the road due to video quality and/or feature improvements, would you keep your modded universal player for audio purposes, or would you also need high quality audio in such a player upgrade? This may impact what you choose as your modded player today.
In my case, I am fine with using the 47Ai for my current video needs. When I'm ready for DVI output or HD DVD, then I'll be looking for a new DVD-V player but still keep my 47Ai for audio (assuming I haven't already upgraded for audio by that time, as audio is my top priority).
Doug