Going back to the schematic for EL-34's cannot be a bad thing...and Marshall does take the NFB wire off of the 8 ohm tap. This needs to be done no matter which method you use.
There are four resistors to change if you go back to the El-34 schematic...the two 220K's in place of the 150K's, one 56K in place of the
47K, and one 27K that needs to be changed to a 15K near the IN4007 diode in the bias circuit.
Obviously, paralleling the 47K is simpler. IIRC, some folks advocate changing the 220K's to 100K's on these Marshalls, so the 150K's might be good as is???
Maybe Tim Swartz will chime in here and share his experience with Marshalls. I would prefer to go back to the schematic, I suppose. The Plexi Palace Marshall forum would surely give you some good feedback.