First question: Why you need to improve that preamp?
Have you high-grade source and amp, much better than this preamp in your system, so that you need to improve the preamp?
Second: maybe a larger photo will be better, to see all the preamp.
Every device can be improved, but it depend of many factors. If you have expensive - state of art - devices in your hi-fi system you need to improve the preamp with expensive components, like paper-oil caps or polystyrene or polycarbonate(for details on caps see
http://members.aol.com/sbench102/caps.html ), alps or noble pot and so on. Also nos Mullard ECC83 are fine...
If you have human devices I don't think you will see differences just improving only this preamp...
Post Scriptum: Tubes work better in vertical position.
My 5 cents-
cheers.
Fabio.