I realize this is a pretty lo-tek solution but what if you just added another switch. Like most tube-based guitar amps use. One is on-off and the other is standby-on. So if you turn the amp on with the other switch in standby, all you really do is supply the heaters so the tubes can warm up. After several minutes of warm-up, you flip it out of standby.
Not much can go wrong with a good switch and their easy to replace. Relays and delay circuits are another matter and can get flaky on you.
