1. Under-biased tubes will produce a restricted, thin sound that lacks any sense of responsiveness. Scientifically (in the lab that is), under-biased tubes will be shown to produce reduced power output, and copious amounts crossover distortion. While 30 Ma will produce reasonably good performance, there is no doubt that your other tubes are clearly not operating on the linear portion of their characteristic curve during the hand-off period between the tubes once Class AB1 operation commences with such a low bias setting.
The control section and buffer modifications address completely different issues than that addressed by balancing output tube current flow and the installation of EFB(tm).
The need for the installation of Screen Stability resistors is determined by the type of output tubes used and how they are operated. Recapping the power supply with modern low ESR caps and the installation of EFB are certainly influencing factors relative to the need for SS resistors, but the main criteria starts with the design of the circuit to begin with.
Dave