Most people can't see those differences they are measured with some sort of instrument, customarily a specifically made taper gauge of some kind or a dial gauge. In addition to the end establishing, other dimensions of the facing also measured with micro able tools give a contribution to its resistance: the contour of the curve among the point where the reed contacts the table and the mouthpiece tip and the true length from the top to the purpose where the mouthpiece contacts the table are the most vital ones. In standard, the longer the curve, the fewer resistant it is. In usual the closer the top opening, the less resistant it is. But these can be mixed in novel ways to balance or confound one another. Also, there are variable dimensions in the baffle, throat, chamber and even the back bore that may materially add resistance or not and make a curve that should give a definite resistance level a whole various feel from what you'd expect. I do not believe there is necessarily a connection between a player's alternative on Bb and his preference on bass. Especially if the bass is a double, reed friendliness and simplicity of transition between the blowing characteristics of the bass setup and your accustomed Bb setup are more crucial. A stiff reed on a close facing doesn't feel the same on a bass as it does on a Bb. You may like an equivalent to the M15 or not, but I don't believe what you use on Bb will necessarily be the average. Don't forget that bass clarinets, even when in excellent mechanical condition, are inherently more resistant than a Bb just on account of the dimensions of the air column concerned.