Hi Rob, To answer a couple of your questions,
Tubes gradually wear out and at some point this will result in sound degradation. Small signal tubes (not power tubes) usually last 3,000 hours, give or take a thousand or so. Some old stock tubes (made between 1940 and 1980) are built to do 10,000 hours.
They can go bad at any time, but with small signal tubes (or preamp tubes as they’re also referred to) they will most likely not cause any damage to your gear.
They can also go microphonic (send feedback through your speakers) if you drop them on a hard surface or something like that. If you buy from a reputable seller they should be guaranteed not to be microphonic on arrival. So handle them carefully after you receive them.
New Old Stock (NOS) can go for anywhere between $75 and $500 a pair. New production tubes are much cheaper, say $25 and up a pair. These are made in Russia and China, often branded with the names of classic old stock tubes. Many people are quite happy with the sound they produce.
Don’t be scared off by the downside of tubes, they are not much trouble or risk to you or your equipment and they can produce really nice sound. The Aesthetix separates you mentioned should sound very good.
The people at Aesthetix would be happy to advise you on tube choices as would the good sellers.
Other good sellers are:
Upscale Audio
Tubemonger
Vintage Tube Services
Jim McShane