There are so many ways power transformers are made. I on all my valve power amps design my power transformer so that a low voltage secondary winding behaves as an electrostatic screen so either mains terminal end of the primary winding behave about the same. The leakage voltage and current you are talking about as I said before is microamps unless there is something very wrong. Capacitance is in the order of 1 nF max. Even Toroids which on small sizes often wind the secondary first have so small a capacitance to be negligable. I mean it is like urinating into the sea, Yes it will raise the level but can you measure it?
If it really is noticable, then try adding a capacitance of say 500 pF rated 630 volt minimum from the one terminal to chassis and then reverse the connection of that capacitor to other terminal. Then of course you have the source component and the power amps all injecting the small leakage current in varing amounts all adding to the common chassis / ground that links all your audio units being tied by the phono or XLR leads.
So go through all your units one by one then colectively change the mains to all units simutainiously.
The preamp your refer to of course is a US product and they do tend to use cheap laminations and run a bit hot due to 60 Hz design. The themselves do not design their own transformers but go to outside contractors.