I love tube amps and have learned that the worst thing you can do with one is turn it on and listen to it. Sure, it might sound great, but that leads to tube rolling from the audio geek compulsion to change stuff, and wear and tear on the tubes. Just put the amp there and leave it off...you can still look at the tubes and feel like you know what you're doing and are part of the tube freak world. But by no means should you actually use a tube amp...it will just lead to trouble.
Would you buy a tube amp if you were unable to use vintage tubes in it ?
Not available or too expensive.
Hmm.., I don't think I have a definitive answer for myself, but I would do my best to avoid such amps. There is no substitute for great tubes, I guess, especially if you value sophisticated sound.