Prices on mid 70s/ early 80s Marshall rigs seem to be on the up. I've seen amps I had trouble selling for a couple of hundred quid in the late 80s sell for £1000+. .
Strats have always sold well, Teles not so much - that may be a good bet for the future.
When people eventually get wise to the Class D sham, classic valve amps will inevitably become more desirable.