all you really need to mod one of these amps (VJ or GA5) is some nice NOS valves and a good quality alnico speaker. I have a couple of different 5w valve amps (one i made my self and one old 60's one). I brought a selection of pre and power amps and experimented till i liked it - no need to worry about re-biasing on these simple little circuits
You gotta wonder why the guy apparently stripping down and "completely rebuilding from the ground up" puts the old logo back on - surely that severly limits what he can charge for what he is claiming is practically a boutique amp