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

It seems like the MA6500 will work well with either load, so it shouldn't make much difference whether you use nominal 4-ohm or 8-ohm speakers... as long as the speakers' sensitivity specs are not too different. Given the same efficiency, there will be more available power into 4-ohms, though it's likely that the better speaker (whichever one it is) will still sound better (unless you are clipping the amp at your preferred listening level).

If they are equal in effencey then there be no difference at the same level, the 4 ohm would have a little more power available and play a couple of DB louder. Neither is preferred in itself. A transistor amp will try to double its output when the impedance is halved. Most don't have the capability, the Mac should be producing 240 instead of 200 but that is not bad. So with lesser quality transistors low impedance's should be avoided, even high quality ones can have problems with VERY low ones, say under one ohm. Since tubes usually have transformers to match their impedance they do not operate in this manner. The impedance derives from the particular design of the speaker and neither is preferred in itself, they are a consequence and not a design goal except to make them usable by the amplifiers in their price class. Typically, the more drivers the lower the impedance.

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.

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.