A good 7 hour listening session today. The Avantic is re-built and re-assembled and sounds pretty good.
I went with the miniature 4 Amp switch in the end - made a small panel out of aluminium so it fits perfect in the AC outlet slot with no cutting or drilling of the case.
One area I didn't touch is the bass/treble circuit. The polystyrene caps look original (60 years old). I mention as I think the mid-range is a little flat compared to my other valve amp. I'll have a few more sessions before I replace them.
