4 ohm load vs 8 ohm load? What is prefered?

With 4ohm you gain 15.7% of perceived loudness but loose almost 100% of damping factor (120 vs. 230). Choose speakers that sound better to you - forget numbers.

Perceived Loudness = k^(1/3.5) where k is a ratio of power.

Casouza - I agree. I believe that great specifications are often a contrary indicator of sound quality. I would, for instance, avoid amps with thd=0.00001% at any cost.

Higher df might be of some value since xover inductor in series with the woofer is in order of 0.08 ohm limiting df already to about 100 resulting in total df=50. DF gets much worse with frequency and might get worse for small signals. It is all, most likely, speaker design dependent.

"should get peaks 3dB higher into a 4ohm load than an 8ohm load."

- 15.7% louder is easier to imagine than 3dB.

"The damping factor is only relevant when you solder the output devices to the speaker terminals and use active cross overs"

- I don't know where you getting this from.

"Of course, with an odd order passive Butterworth electrical filter the damping factor will be just 2.4 at the cross-over point and even order Linkwitz-Riley filters will have a damping factor of 1"

- we are not talking about filter's "alpha" but about damping factor defined as ratio of speaker impedance to amplifier's output impedance and yes it is important up to some point. I'm trying to show that it is limited for woofer by resistance of inductor in series to about 100 and would be nice not to lower it even more.