The transformer I have in mind is slightly short on voltage which after rectification would leave me about 20volts dc short of the spec
250 +/- 20 is perfectly acceptable. If the preamp you're looking at has a 12AX7 with a 100K plate resistor and a 1.5K cathode resistor and 250 volts B+ then it is all boiler plate. These things are very tolerant of variations in values. If your quiescent plate voltage is somewhere near half the B+ then you're good. Because the triodes are cathode biased, it takes alot to upset them. Don't be afraid to experiment with values.
Have you seen duncanamps tone stack calculator? Have a look at the tone stack you're considering and see if you like what it does in simulation.
I prototype things like this:
and when I'm satisfied I'll find a chassis and build it like this:
Don