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.

Kijanki, I doubt that anyone can pinpoint the difference between a damping factor of 120 vs. 230.
Only poorly designed, very boomy speakers require a damping factor above 20. These are just numbers, as you pointed out.
The OP should decide based on listening tests.

01-12-10: Kijanki
>With 4ohm you gain 15.7% of perceived loudness but loose almost 100% of damping factor (120 vs. 230).

1. FTC amplifier ratings are an over-reaction to the 1970s amplifier spec wars which have little bearing on actual music. A nice jazz recording has 18dB between average and peak power. FTC power ratings are five minutes of sine waves with 3dB crest factors. That's preceded by an hour of "preconditioning" with a 1KHz sine wave with average power 1/3 rated power (prior to 2000) which is actually the hard part (they reduced it to 1/8 power).

When you plan to use the amplifier for music instead of science experiments you should get peaks 3dB higher into a 4Ohm load than an 8Ohm load.

2. The damping factor is only relevant when you solder the output devices to the speaker terminals and use active cross overs. Keeping the active cross-overs but adding 8' of 12 gauge cable will drop the numbers to 83 and 42.

In the 4 Ohm case you'll have at worst a .2dB peak at the bass driver resonance which is generally believed to be inaudible so it doesn't matter.

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. Moving into the stop band the damping factor will be less than one.

If you think damping factor is important you need to stop using passive cross-overs instead of worrying about amplifiers.

>01-12-10: Rsfphil
The way I understand it is given the same sensitivity the amp would have to double down into 4 ohms for the 4 ohm speaker to play as loud as the 8 ohm speaker. The MA6500 almost gets there so it would be very close. I doubt if anyone would even notice.

If the speakers had the same _efficiency_ that would be the case although most speakers are rated using _sensitivity_.

A 90dB/2.83V/1 meter speaker will play 90dB measured a meter away with a 2.83V RMS (4V peak sine wave) input regardless of its load impedance.

That will take 1W into 8 Ohms or 2W into 4 Ohms.

A 90dB/1W/1 meter efficient speaker has 90dB/2.83V/1 meter sensitivity with an 8 Ohm nominal impedance and 87dB/2.83V/1 meter sensitivity when the impedance is 4 Ohms.

Sensitivity is the relevant number unless output transformers with different taps get involved, since music which doesn't make your ears hurt from the volume has average power so far below the peaks you're just looking to keep those from clipping.