I can't really comment on the preamp design. It's a 4-stage gain machine, and looks good, with the possibility of minor tweaks after construction to make it sounds exactly the way you want.
I agree with PRR that the bridge rectifier along with 190-0-190 or 300-0-300 is confusing. If you plan on only using 1 side of a PT secondary (i.e., only from CT to one outside tap), you could do that, but you can't draw any more current than the transformer was passing when using the whole winding with a full-wave rectifier. So 190 -> bridge = ~270v, or 100-0-100 -> bridge w/ CT not used = ~280v.
You may want to ask the original designers if 270v for the preamp helped the character of the distortion.
Oddly, my biggest concern is that the effects loop is not really practical or functional. It's a good idea to have it at the end of the preamp chain to capture the sound created by the preamp, but needs to be knocked down to about 1/200th the size (guessing 20v to about 100mV) for feeding into pedals meant to gulp guitar-signals, then will likely need to be boosted back up to drive the output tube.
I think I see why you asked about 6L6 ratings/cathode resistors at 270v in the other thread.
How close can you get to 300v supply? That requires rectifying about 200-215v, and whether you use a full-wave or bridge will be determined by whether you have a 200v secondary or a 200-0-200v secondary. Assume the plate and screen will both be at 300v. For 5881, 25w/300v = 83mA, and using the triode curves (triode operation is with screen connected to plate, so both have the same voltage on them), you get 80mA with 300v on the plate at 20v of bias. Let's guess 3mA screen current at idle based on some of the data sheet conditions, so 20v/.083A = ~241 ohms, so 250 ohms is the closest standard value. Dissipation in this resistor is 1.6w, so pick a 5w resistor. Cheating from the data sheet, optimum plate load looks to be anything from 4-5k.
20v of bias means you want the preamp to be able to easily deliver at least this much. I plugged the tonestack values into the Tone Stack Calculator, and the average loss for settings midway up seems to be about 10dB, which is a reduction to about 1/3 the incoming signal. So we need at least 60v of swing from the stage before the tonestack (OD channel) and preferably more. The stage ahead of the tonestack looks to be plenty capable of doing this, and all the interstage volume pots should help you set an overall gain you like.
All-in-all, it looks good. I'd be mainly looking to be sure the PT and power supply were right, or simply grafting this preamp onto an existing amp's power section. The 269EX doesn't have the current rating to power a 5881 or 6L6; it is rated at 71mA and we'll need more than that at idle. Hammond does have a 200v CT (100-0-100) transformer rated at 115mA, but the only filament winding is 5v 2A. But they also rate their products for 115v on the primary. If your wall outlet puts 124v on this transformer, you get about 215v CT and about 5.4v on the filament winding. For the high voltage winding, ignore the CT and tape it off, and and use a bridge rectifier; for the filament winding, the voltage is about 14% low but you also won't draw the full 2A rated for the winding. The voltage will probably end up a little higher than spec, just because you'll only pull about half the rated current. Either way, it will light your tubes and work just fine. The transformer is the 263AX and I've seen it priced around $40 plus shipping. I did not see a comparable Edcor transformer that didn't have a 230v or 240v primary.
