If you have not already Tube Rolled the Input Stage on the Pre-Amp and Power Amp, there is a trick being missed.
A Investigation on Tube Options and trials may just deliver what is being sought.
I would make the best Tubes available for the Pre' as the better the signal is processed at this the device is for the benefit of the Power Amp.
Trying different Umbilical's at all interfaces can be tried out, these being selected for their allowing New Tube selection to really show of their magic, will be a very cost effective way forward.
I can't say enough good things about PC Triple C wire used to produce Umbilical's and Speaker Wire.