Coupling Cap Values for a VTL ST150?

Usually the value is .47uf but guess you should check with the ones that are being used in you amp.

As for what type of cap, Jensen Paper-in-oil caps tend to be more musical, a bit roll off top, euphonic sounding, while Hovland caps tend to be more neutral but still very musical, detail with sweet treble.

Your experienced modder should be able to provide this information. If he can't then you should keep looking.

Here's one person's opnion http://www.humblehomemadehifi.com/Cap.html