Best upgrade option for this system?

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 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.