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.