Warmth - what is it? How do you get it? Nostalgia - it isn’t what it used to be (1)
To see the look on some people's faces when they see an old piece of audio gear you might think that they had bumped into a long-lost friend or their first love from eons ago. It is reasonable to ask whether audio equipment might improve with age like a violin. Actually, it is thought by some that violins don't improve with age, we just don't have the knack of making them like Stradivari and his chums did. Guitars don't get better with age either - they just acquire 'character', and to compare an old hand-made instrument with the output of a modern production line is not necessarily comparing like with like.
Electrons neither improve nor mellow with age, although components certainly change in their characteristics. But as components age, we are more likely to find sonic artifacts that qualify as fault conditions rather than the warmth we are seeking. Bring the unit up to its original specification however and the purist will claim that it's not the same and something is missing.
I think it is fair to say that there isn't any intrinsic value in the age of audio equipment - probably most so-called vintage gear was probably regarded as not being particularly good in its period of currency, and it won't be any better now. But if a piece of equipment did once have a nice sound, then there is no reason why it shouldn't still, once renovated if necessary, deliver the goods - at a cost in convenience and often sheer size and weight.
Looking inside the vintage gear that is cherished by many, we see the true object of our affection - the good old-fashioned vacuum tube or valve. Doesn't it have a lovely warm glow? This is probably where the notion of warmth comes from, that valves actually are warm - quite hot in fact - and the equipment has a characteristic aroma (I'm surprised that someone doesn't bottle it as a fragrance and sell it to die-hard sound engineers). It has to be said however that the reason valves were invented is because no-one had thought of transistors yet.
There are few areas in which valves are technically superior to transistors and none of them relate to audio. (If you want to know, valves are used for very high power applications and they are more resistant to the electromagnetic pulse generated by a nuclear weapon). In fact valves have a very major drawback in that electrons can only travel one way, from cathode to anode. Efficient transistor amplifiers can be designed which use an npn transistor to push electrons one way and a complementary pnp transistor to pull them the other. To do this with valves means a highly asymmetric design with which it is difficult to handle the positive and negative halves of the signal waveform identically.