Adcom GFP-750 compared to AR LS-15

Ive had the Adcom for ten years now and Im fairly happy with it. In passive mode it is smooth but has some loss of dynamics. In active mode its solid state signiture comes out on some recordings. Other preamps Ive had are Tandberg 3008A and 3018A. I have a bryston 14BSST amp. Ive been looking at tube preamps and want to know if the Audio Research LS-15 would be a significant upgrade? I know it listed for $3000 in 96-97 and the Adcom listed for $1500 in 2002. I also have for a phono preamp an Acoustech PH1-P and new OHM 5000 speakers which are excellent and revealing. Would the phonostage in the AR be about the same or more liquid but not as detailed as my phono preamp? And if I find an AR LS-15 without a phonostage can I retrofit one without sending it back to company? I listen to records about 70% of the time. And one last thing would the AR be a suitible match for the Bryston for gain etc.? I do want a tube preamp because my sound is just a little bright. I am a very critical listener so it might be fine for some. Concerning the Adcom preamp. It was a class A product in Stereophile in 2002. If it was such a great buy, then why hasn't the company continued to make it? Their new model GFP-815 lists for $750 and is on sale now for $499 at their website. Could their new model be an improvement over the older GFP-750? I appreciate all opinions and suggestions. The members here have given me a wealth of information here in the past and Im grateful
The ARC LS-15 is a line stage only and no you can't put a phono module inside it.

Yes, the LS-15 is several steps above your Adcom. The GFP-750 is a nice unit, but not in the same league as the ARC...IMHO.
And I would second that opinion having owned both.
Its almost impossible to give you very specific answers to your questions without having the equipment on hand to listen to. One thing I can tell you for sure is that a passive preamp can eliminate a lot of problems that you will encounter with lesser active line stage. The preamp is extremely important to get right if you want good sound. The biggest mistake people make in audio is not taking the preamp as seriously as they would other components, like amps or speakers. The advice I would give you is that unless you can buy a really good preamp, tube or solid state, keep your passive. If not, you'll be having all sorts of fun buying cables, tube rolling, conditioning your AC, etc., in an attempt to fix that something with your system that you just can't seem to put your finger on.

Just to clarify, you mention a touch of SS signature when you switch your 750 into active mode. Thats what I'm talking about. Its not so much that its solid state, its just not good SS. Your first instinct is to get a tube preamp. In my opinion, thats not the best way to handle the situation. I think you should focus on getting a GOOD active preamp, not on getting a tube preamp. If it ends up being SS, thats OK. If it turns out to be tube, thats fine as well.