A loaded truck driving up a mountain has to downshift gears to enable the engines torque(power) to be used.
Its a difficult task (load) and more driving power is needed.
Once the truck gets over the hill and is coasting down the hill very little power is needed as the load is lightened by gravity.
With a preamp, power is related to low output impedance , but its relative to the load (power amp) it has to drive.
If a power amp has an input impedance of 10,000 ohms, then an ideal ratio of 10:1 would mean a preamp impedance of 1000 ohms.
10,000 ohms is a typical figure for a solid state amp, but a typical figure for a tube power amp is 100,000 ohms, so applying the 10:1 ratio the preamp could have an output impedance of 10,000 ohms.
10:1 is like the ideal gearing in our truck analogy, the power can be transmitted cleanly and without strain.
There's no loss of gain and frequency remains linear .
Its possible to get the output impedance of a tube preamp down to 100 ohms or less, which would drive the most difficult solid state preamp, but there can be sonic compromises to this .
An output impedance of around 1000 ohms is ideal for a tube preamp, as it can be feedback free and still capable of driving 95% of solid state power amps.
Things get a bit more complicated when we look at the impedance issue with power amps driving loudspeakers.
If a speaker has an impedance of 8 ohms then applying the 10:1 rule we need a power amp with impedance of 0.8 ohms.
No problem for a solid state power amp, but a tube power amp which, without feedback , will have a higher output impedance will not be able to achieve this ratio.
The problem is compounded in that a loudspeaker doesn't have a constant impedance - it varies with frequency and can shift from less than an ohm to a 100 ohms, which is worse case, but they all have varying impedances.
Use a Single Ended , no feedback, tube amp which will have a high output impedance and the resulting frequency response will be far from linear.
Which is why many people like Single Ended amps, they act more like a tone control, accentuating the mid range and sounding "romantic" .
Single Ended amps are best with easy loads, like crossover less full range drivers, or used in bi/tri amp systems, driving the midrange or treble drivers.
Push Pull tube amps, with a little feedback,are much more capable, but still need a loudspeaker suited to their capabilities.
Fortunately the preamp/poweramp impedance issue is a lot more straight-forward , and you really can't get better sound than combining a capable tube preamp with a tube or solid state power amp.
Here's a measurement of a single ended tube power amp with output impedance of around 2 ohms - the black line is frequency response into a simulated speaker. Its actually not too bad for a SE amp, but it's far from linear, and will sound coloured.
