Plenty. The power supply is the most important part of an amp.
I'm not too much up on solid state, but consider what it does in a tube amp. If you're not supplying enough power to the tubes, they'll sag or clip when the music hits a transient peak. You'll hear that. You also want well filtered and regulated power. When power comes off the transformer (which should be big enough to supply more than the tubes can draw), it needs to be rectified from AC to DC. Not all methods are equal - in less expensive, simpler, power supplies, you still get some AC ripple. You'll see a wavy line instead of a flat one on a 'scope. You can get away with some of that, but that also means there's some fluctuation being injected into the circuit. To get the DC really smooth, a perfect flat line, you're going to want to add chokes and big capacitors to smooth it out.
That, of course, adds cost to an amp. A choke will run you $50-$100 or more. Chokes and big caps also use a bit of power to work, so that means you have to spec a bigger power transformer to compensate. That also increases the cost. You'll probably want to add regulators to the power supply, as well. These will ensure that a regulated, consistent flow of power goes into the circuit. Again, this adds cost.
I should add that you can rectify and/or regulate with solid state or with tubes. If you choose tubes, however, those tubes will need yet another power supply for their filaments. You might need to use a second or third transformer just for those.
Aside from supplying power to the circuit itself, you also have to supply power to the tube filaments. Those are sort of like an internal lightbulb for tubes (they light up
) that maintains a consistent temperature (tubes have to be warm to work), which aids in electron transmission.
Now, you can run regular AC to the filaments. But if they're responsible for electron transmission, if they vary, they can also make the signal vary. This is a small effect, but it can make a difference if you're looking for precision and the blackest background. That leads to a couple of other things - if you're running AC filament wires around the signal path (necessary if you run AC to filaments) then that AC can radiate into the signal. There are ways to counter this (e.g. shielded cables and twisting the filament wires) but you still have to deal with potentially noisy AC.
So in addition to working over the main circuit voltage (aka B+, if you've seen that term), you can use all the same techniques to clean up the filament power. Of course, this means the additional expenses all over again.
That also means using a bigger chassis and substantially more labor to assemble the amp.
If you look at the difference between inexpensive and more expensive tube amps, you'll almost always find the difference in the power supply. The least expensive ones will have a marginal transformer that runs hot rectified by some cheap diodes and then fed right into the circuit. On the other hand, one selling point for the Zana Deux to me is that it had four chokes inside its substantial and well-considered power supply. It wouldn't be anywhere as stable and authoritative without that, though it could be built for less than half its price with a cheap supply.
Lots of people focus on tuberolling as a desireable feature, but it's the power supply I consider first. My philosophy is to have the best power supply possible run a very simple circuit. That, in my opinion, is the way to the best sound. I think the simplest possible circuit run with the cleanest, best regulated power, gets you closest to the music. There are other important design considerations, but the power supply might be the most important.
