Suppose we have poweramp A, 2 * 50 W solid state
and amplifier B, 2*50 W tubed.

First try:
B costs 4 times as much as A, running costs (electricity,
retubing) are much higher, mtbf is worse...obvious, A is the
amplifier with the better price/performance, no ?

Nope, flawed reasoning !
to calculate the true costs, we'll have to take into consideration, that
in case an amplifier fails, this will bring costs that should be taken
into the running costs equation. If the solid state amp happens to
fail with loads of DC on the outputs for a few seconds - speaker
mayhem !!, so that is a hidden, but very real cost. From this you can
see, that what is actually required is a risk analysis of different modes
of failure and the costs involved there. Then there are other hidden
costs: are parts still available for repairs or is a costly
re-engineering
of the amplifier required ? Etc. etc.

Second try: we have taken the previous considerations into account, now
with the proper running costs calculations: B costs 4 times the price of
A,
but running costs for B are half those of A :-) Not a clear cut answer,
now!

still, this is not enough. we haven't even begun to specify, what we
actually
mean by *performance*, so far, implied, only maximum output power came
into the picture. Here are some other criteria that someone could fit
into
performance, weighing factor for each is an individual matter !!
distortion level & spectrum in the 0.05 to 5 Watt output power range,
damping factor, heat generated, looks, ease of use, etc., etc.

Life is about choice,
Rudy