The A-T W3000ANV appreciated nicely (> 30%) if you bought them new. I suspect a BHSE amp will do the same when production stops. And if Stax ever goes away (perish the thought)...
[Or if Spritzer stops crafting KGSS-family amps...] There are no readily available substitutes for these things.
[Edit: not to mention the R10, but that's too obvious.]
It depends on the popularity as much as the performance though imo. For instance Apex (Pete Millet) amps are as rare as you will get. Just an estimated 50 wheatfields and only 40 peak/volcano's. Still took me a while to sell my wheatfield ha-2 and I have had my peak/volcano for sale for a long time now and no takers. Half price .
R10's, KGSS, BHSE, Mjolnir, W3000anv., Orpheus etc. have rarity as well as popularity going for them. Trust me when I tell you Apex amps are every bit as good as performers as the pieces of kit you mentioned. Just not popular for whatever reason. Flooded marketplace imo.
Edited by jibzilla - 10/18/15 at 2:24pm