Regardless your preamp uses output coupling cap you would still need a positive supply. Positive supply compensates the negative DC voltage at the amp’s input. Theoretically if you do not connect or disconnect the cable between your preamp and power amps then you would not need the positive supply as it will be grind on the preamp output. However, even if your preamp has ultra low output impedance (a few own, that is very unlikely with capacitate coupling) then still you will have some auditable click via loudspeakers what you change value on your preamp. It will be due to the present of DC at the power amp input. To compensate it with a positive supply is an elegant and comfortable solution.
You might subtract a positive voltage via a deviser from B+ but since it is not regulated it will be drifting with main fluctuation. In a past Dima and I made quite a few experiments to find a correct solution. Dima design quite slick circuit that monitored the voltages drift and automatically composite it. The problem was that since it dealt with grid everything we touched affect sound very dramatically, always in negative way. The gas regulator tube in negative supply, in the way how we use it in the Milq did not affect sound negatively, but quite in contrary it affected it superbly positively. From there to put in the game the same positive supply was not complicated. Do not think about the gas regulators as the regulators but rather as a decupling devised that provide the necessary compliance of the bias PS to the grid current’s rollbacks….
Generally, having the Milq up and running and contacted it to the preamp it is possible to shut down the positive supply. Then you might need it on only if you need to connect or disconnect the cable. This increases current through the cable is also works very beneficially. But I would not advocate this mode. I would do it myself, but would not suggest if for others. I did not detect any sonic advantages to do so, still if I wish to do it than I plug the cable and put my Cat to guard the cable, making sure that no one would disconnect the cable while the amp it running.
Romy the Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche