"I want to ask a question but have no technical knowledge so forgive me if it appears stupid."
I have to stipulate to the same ignorance but will not be able to beg forgiveness after what follows :-)
"It seems to me that the finest sounding products are DC powered. Kevin's PS1200 for the SL 1200..."
The PS1200 is what any normal audio power supply is: the part of the circuit that takes 60Hz, 120v alternating current from the wall and converts it to direct current that the component can use (simplified account). That description applies whether the power supply is located inside the component chassis or is housed separately, a not-uncommon configuration though still in the minority overall (excepting high end turntables). So it doesn't mean much to decribe the PS1200 - or the SL1200, with or without the PS1200 - as being "DC powered", because in that sense all gear is DC powered.
"Why couldn't all audio products be supplied...with a wall wart...? This removes the noisy transformer away from delicate electronic circuits."
It's true a wall wart removes the transformer from the chassis (as well as potentially limiting its size), but not the remainder of the power supply. Anyway, for most components not as sensitive to vibration as a turntable (and in truth that's all other components, despite audiophile obsession over the idea), with a little care taken in design and implementation the complete power supply is built into the single chassis with near-zero negative effects (and even some positive ones) - just witness the vast majority of high end gear.
But striving for the last squillionth of refinement, some all-out component designs do segregate the entire power supply (and frequently the non-audio control circuitry as well) in a second box. This may or may not be as effective as claimed compared to a single box, and may or may not be duplicable in a single box in terms of sound quality - after all, there's usually no real way of the consumer knowing for sure.
(The situation of the SL1200 which Kevin exploited in introducing his PS1200 is unusual in regard to high end turntables generally, which would never have this sort of stuff onboard to begin with, and now we SL1200 owners can appreciate why.)
"An obvious upgrade would then be a large transformer, mains powered with several DC outputs that could be used to replace the cheap wall warts."
Upgrade from wall warts, potentially yes. Musical Fidelity markets its X-PSU under this theory, but notice that their real high end gear contains either onboard power supplies or complete dedicated power supplies housed in second chasses. The power supply is one of the most important sections of any good piece of audio gear, and a designer needs to have a free hand to approach all aspects of a component wholistically, so one-size-fits-all is not a practical approach for more ambitious products.
Battery power is of course different (though not necessarily always, or in all ways, better), in that it requires no transformation from AC to DC or the attendant needs for isolation and smoothing the ripple. And you sever the connection to the powerline grid and therefore the noise and distortion that contaminates it, even though batteries can have their own practical difficulties and limitations. My own phonostage happens to be battery-powered, the application that's always seemed to me to be the easiest, and likely the most sonically productive, for which to implement the technique.
