Check valve or not, it's still muffler pressure; once the regulator opens and fuel flows, whatever pressure exists in the tank is reduced or exhausted unless adequate muffler pressure exists and can keep up with the pressure loss due to fuel flowing through the carb- which it may or may not depending on the type of exhaust system being used. As restriction diminishes, so does pressure- a basic tenet of fluid dynamics. So... You utilize a positive pressure means such as a Perry pump or crankcase pressure (crankcase pressure necessitating the need for a check valve to accomodate the positive/negative pressure pulse in the crankcase) which will provide a consistent pressure source which the regulator controls dependent on fuel draw (negative pressure seen at the fuel feed inlet) through the carb.

This isn't intended to be arguementative or otherwise but it seems there's a lot of mystery surrounding on-demand regulators which both the IB and Cline are- direct decendants of the Walbro system with the IB having a few design changes but still operating under the same principles. When looking at pressurized fuel systems, I want something that will provide consistent performance regardless of tank location, airplane attitude or fuel level; muffler pressure won't provide that (well, at least it never has for me).