What sort of speaker are we talking about??
... a few points, the cap's effect is WRT the impedance of the speaker - it has little to do with the output impedance of the power amplifier, tube or solid state.
Bear, The speaker is a RS 8" in a wooden sealed box with foam. I just looked at similar RS speakers on their web page:
http://support.radioshack.com/support_audio/doc15/15880.htm I'm not sure if mine is the same, but it is similar. The VC inductance around 1mh caught my eye. The parallel L-C resonant frequency with a 10uf cap is also 1600 hz!
I repeated my test with and without the 10 ohm series 'R', and the 10uf across the speaker ONLY makes a difference with the resistor in place. Sorry, but the amplifier Rs does matter.
Sooo, take a amplifier with a zero ohm Rs, put 10 ohms in series with it to the speaker, and shunt the speaker with 10uf. The R-C by itself form a 6db attenuator at 1600HZ, and the VC inductance with the 10uf have a parallel resonant point at 1600 Hz.
All of a sudden this is not so simple....
Jim
WD5JKO