pdf64 hit the nail on the head. Never build for a musician "in search of...". You will not get paid for your efforts and most likely end up with the amp at the end of the painful journey.
There is clean - sterile clean and if that's what he wants, buy a SS PA head and upcharge him 20% to cover you time. Sometimes when a player says they like clean clean clean, they actually mean warm. This probably due to some sort of PTSD with a Bogner Uberschall or something similar with todays cascaded preamp buzz.
The Hughes and Kettner Puretone has peaked my interest ever since someone posted here asking for a schematic so he could do a repair. I had no luck finding one, but I did listen to a few youtube posts and reviews. Everyone raves about it's clear tone and how touch sensitive it is and I mean they RAVE! Soundclips are impressive. Very full bodied and rich. I was very surprised for a class A amp with two EL34's. Due to the massive amounts of headroom, people say it is much louder than the specs suggest, although it is supposed to kick 40 watts into 8 ohms

. Single channel, simple tone stack, what could be better! AND, supposedly the "Growl" control gradually bypasses the tonestack if you need a little more bite. Point to point construction. Boy it would be cool to find the schematic for this....
Bottom line, have him show you or find an amp that meets his needs and go from there before you start ordering parts.
Jim