Since speakers are not a consistent impedance over there whole frequency range you really have to try both taps and see which you prefer.
Yes I heard about the damping factor. My bass is really tight for an old tube amp, and I wonder if the system will be less so on the 8 ohm tap. I'll try it tonight and post back.
BTW I read on some site that Sid Smith and Saul Marantz disagreed about whether there should be a low frequency filter built into the 8B to filter out turntable rumble, so they compromised and made the "test" outputs without the filter (as Saul wanted) and the normal outputs with it, as Sid Smith wanted. So the "test" outs are the ones to use.
Yes, they did it by choice. However, you can see some significant differences in this test done by Stereophile. It really shows in the power output differences between the 4, and 8 Ohm output power measurements. Without the other impedance taps, we'll never know how it would compare sonically, to its one fixed output.