I stand by my suggestion that experiencing a Valve Exchange, could prove to be all that is necessary to discover the sonic that is more of an attraction to yourself, and even possibly an attraction that comfortably fends of other Phon' Models. I am not seeing where the method fails to be a experience that could prove to being cost effective, as a means to make a change.
For myself to have my E88CC Phon' superseded, it took a Bespoke Built Valve Input/Valve Output design to achieve this. The E88CC is still used and has proven to be my most preferred with a Head Amp coupled to it. It is not one that is a relegated to being a 'sell on' item or to being a 'storage' item either.
The CCa referred to earlier was a Matched Pair demo'd in my DAC, but the Siemens & Halske early 60's E88CC's were the ones selected over all Tubes offered as a loan for the Tube Rolling experience.
Early 60's Mullard E88CC's were selected for the Phon' over the ones used in the DAC and all others loaned.