I was having a few problems with a P-retro I'd bought secondhand here and fitted myself - couldn't eliminate a grounding hum. I got in touch with John and he patiently walked me through several potential solutions, one of which cured it - while he was waiting to be seen at hospital! But then it all packed up completely and I sent the whole thing off to him for repair. It needed a new battery unit which, once he had them in stock, he fitted almost by return and at cost price. He also supplied and fitted an LED bezel to the scratchplate for nowt.
So, yes, not only does John make the best pre-amps going but he is an absolute diamond. I have them fitted to four basses and would never, ever, think about getting anything else now.