Best upgrade option for this system?

My system consists of:
Adcom GCD-600, Pioneer DV-578-AS, Adcom GTP-400, Bryston 3B & Mirage M7si. I'm itching to bring some improvements to my system. Which component should I start with?
I think your pre amp is the weakest link. I almost fell to the flor in awe when I changed my old Adcom gfp 565 out for an inexpensive passive pre.
I think that trying a good tube preamp would be the thing I'd try first. Swap the Adcom GTP-400 for a nice tube preamp like a TAD-150 Signature and I think you'd be amazed.
I used to own a GTP-400. It is most certainly masking the sonic capabilities of your Bryston. I'd try a different pre and use the GTP-400 as a tuner.
I can't disagree with upgrading the preamp. Your amp looks to be the strongest part of your electronics. This is not a good thing, because not only is it missing what it can do by not getting better musical information sent to it by the source and preamp, it is also actually revealing the weaknesses of other two in an undesirable way. Always better to have your source and preamp/phono stage outclass your amp than the other way around.
I suspected that the preamp was my best upgrade candidate. I'm not familiar with tubes and will include them in my evaluation. I've noticed from my initial reasearch that not many quality preamps have tuners, and I like Creeper's suggestion to use the GTP-400 as a tuner source. But I would like to find a good preamp with volume remote if possible.

My local dealer is suggesting that I come in to his shop with my Adcom preamp and cd player to run a comparisson test with his Bryston BP6 or 25 and Conrad Johnson PV10. Not sure how valid the test can be since my Mirage speakers wont be involved.
The test will be valid enough for a rough idea. If you like what you hear in the shop, try to arrange to take the Bryston BP6 or CJ PV10 home to find out for sure how you like it in your own room and full system.
I'm also looking into the Moon P-5. Any thought on how well this unit would marry with the Bryston?
Not that familiar with the equipment you've got, though I've owned Adcom in the past. With this said, I'd invest in a good integrated tube amp, Almarro or PrimaLuna. You'll be very much surprised and, quite likely, won't want to return to SS. I never believed this could be true, even for me, until I decided to jump into tubes.
I ran the comparrison test between my Adcom GTP-400, Bryston BP6 C and Conrad Johnson 14LS2. The difference between Adcom and Bryston was astonishing, made the highs much more inviting and brought out the midrange nicely.

The move from the BP6 C to the CJ 14LS2 was more subtle, but I did like the way it held the low frequencies lower and seemed to widen the soundstage.

I fell for the Conrad Johnson! But unfortunately it's a bit out of my $2000 enveloppe. This was my first audition, I plan to keep researching and continue testing until I'm ready to take the leap.
Audio Research seems to be coming up as another interesting tube preamp option for my Bryston 3B ST. I've seen references to the LS-3 on A-Gon, but when I look further, there are good reviews on the LS-15 and 16. I also saw references to SP series (not sure what the difference is). In my case, I would require a remote and I dont need phono.

Any thoughts / suggestions?