I've said from the beginning in my first post that if the problem is equipment related that I would look first to the preamp. I stand by this recommendation. I think that its worth stating again, though, that you need to look for the RIGHT preamp, not a tube preamp. If the right preamp just happens to be tube based, that's the one I would get.
If you start with the intent to FIX a problem by throwing things like tubes, cables, AC products, etc.. into the equation, you will fail. Any time you don't use products as they were intended to be used, in context, you can't possibly expect them to do good in your system other than by luck or accident. Heres an example. A lot of people mention that you should switch from AQ to Cardas cables. Why? I have absolutely no idea. The right way to correct the problem you are having with regards to getting stringed instruments right has nothing to do with cables. In this case, cables can't fix the problem. If cables can't fix the problem, the only other thing you can try to do with them is cover the problem up. By installing Cardas cables in your system, you can hope that the cables will work in a selective fashion, targeting a frequency range that just happens to be a problem area in your system, and smooth the problem over for you; by design. Anyone who thinks something like that can work needs to give Cardas a call and run the idea by them. In all likelihood, they would tell you (while laughing), that would be a bad idea and that they design their cables to have a frequency response that is as flat as possible; just like everyone else does. I'm not saying here that cables don't sound different from each other; they do. The point I'm trying to make is that if you use cables to transfer the audio signal from component to component, you will probably be happy with them. If you get them for some other reason, you won't be happy.