Why does my reese bass sound different at higher notes?

Your reese bass sounds different at higher notes because the phase beating between the two detuned saw waves accelerates as the pitch increases, making the characteristic wobble effect happen faster. I explain this in my drum and bass course: the phase beating you hear (that wub wub wub sound) is actually phase cancellation caused by two 180-degree out-of-phase points within the waveform, and when you go up the keyboard, it's oscillating faster, so it beats more rapidly. The amount of detune you use controls how fast the beating happens, but going higher in pitch will always speed up that movement.