I know from experience, replacing power tubes randomly like myself and most people used to do, resulted in short tube life.
Exactly what bass tube amps did you use for those first fifteen years?
Fender Bassman 50 (blackface) with a 2x12 cab
Ampeg B15S Portaflex with an Altec-Lansing
Ampeg V4B
Ampeg SVT - original no-fan model
I didn't make this stuff up - this was common sense when I was growing up. If heat weren't an issue for tubes, there would be no warm-up or stand-by switches on tube amps, and they wouldn't tell you to put it on stand-by for a few minutes before turning it off or on. Treat your tubes carefully, they will last a long time; if you don't you're asking for trouble. If they are ice cold, do not turn the amp on at all, not even on stand-by, until they are room temp, or at least close. After a long hard run, use the stand-by procedure, then shut it down, and do not move the amp until they have returned to room temperature, or close to it. Common sense. Slam them from a red-hot 4+ hour gig immediately into the trunk of an ice-cold car in the winter - recipe for disaster. And going the other way as well. Common sense. Or you can spend a whole lot of money on replacements, and just put up with really bad sound and no clean power at all from your tube amp. And set yourself up for complete thermal meltdowns, esp. with vintage Ampeg SVT's.
Or you can listen to a self-proclaimed Psycho who probably just likes nothing but distortion from weak, blown, mis-treated, worn-out power tubes.
I love tube amps, but just got fed up with their idiosyncracies. And yes, the Ashdown looks amazing - if my ancient back could only take it. That and a Berg 6x10 or the new 4x12 would likely be just stupifyingly good. Mark Gooday and those guys are just superb; had some dealings with them back in the mid-90's during my Trace days.
And I almost bought a Hexa-valve once....love that thing.
Cheers,
Cameron