There are lots of vises around for the purpose of holding PCB's for soldering (try Panavise for example). To be honest I prefer to have the board sitting flat on the bench where I can get my magnification lamp over it if need be. There is a great jig that someone made up from metal and also some wooden fixtures around for holding and rotating the entire chassis for assembly and debug. You'll have to search around. I prefer to jig and assemble the amp outside of the chassis like this... Different strokes!
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I've always prefered 10" speakers on guitar BTW. I have the standard Jensen P10R's in a Bandmaster repro, and recently got to try a Fender Prosonic combo with 2/10" Celestion's. Loved it
Joe G