simplified version? yes.... with 10nf in parallel with 64 ohms. the 64 ohms remains 64 ohms at all frequencies (we'll ignore the frequency dependent characteristics of the speaker for the moment, and treat it as a resistance) and call it the load resistance (Rl), but the reactance of the capacitor IS frequency dependent, and it decreases as frequency increases. at 100hz, it's almost 160k ohms, at 1khz it's 16k, at 10khz it's 1.6k, at 100khz it's 160Ω, and at 1Mhz it's 16Ω. so until you get to 100khz, it hasn't much effect, but as it's reactance decreases, it shunts more AC current away from the 64Ω load. the point where Xc and Rl are equal, is about 250khz. since half the current is shared equally, the power dissipated in the 64Ω speaker is about half what it was at any audio frequency. this is the "corner frequency". so for the load, this parallel arrangement has the effect of a low pass filter for the current through the speaker.