4 ohm taps or 8 ohm taps?

Actually, I think there is a better consensus (though far from a perfect consensus) on this question than on many other issues in audio. The consensus being to go with what sounds best. And no, you won't hurt anything either way.

Re question one, there are many reasons why it is possible, led by the fact that the speaker probably deviates considerably from 6 ohms at many frequencies. Also, the higher output impedance of the amplifier on the 8 ohm tap will accentuate the effects of the variations in the speaker's impedance vs. frequency curve, and will also result in a lower damping factor and therefore lessened bass control. Also, the plate circuits of the amplifier's output tubes will see differing load impedances depending on which taps the speakers are connected to, with resultant effects that will vary depending on the amplifier design.

The bottom line is that the amplifier-speaker interface is complex, and both components differ from idealized models in many ways, meaning that the only way to reliably predict what is best for specific components is to listen, and perhaps to compare notes with others who have used the same or similar components.

Regards,
-- Al

I found an interesting forum response at this url......http://en.allexperts.com/q/Audio-Systems-835/matching-amp-speaker-impedence-1.htm

Al's response covers almost all of the substance on the impact of different taps, save one issue:

Some amps have "good" sounding taps and "bad" sounding taps, regardless of the speaker load presented them. Variation in output transformer design and manufacture can be material, varying from model to model and even between examples of a given model. From your post ("tinny" treble), I wouldn't be surprised if the issue turned out to be sub par 8 ohm taps on the Ming Da.

Good Luck

Marty

BTW, the issue of manufacturing variability extends beyond taps (though it's less commonly a real problem, IME). If you test enough pre-amps, you'll eventually encounter it. Two different line level inputs on the same pre should be identical sounding, but -sometimes- this isn't the case. IME, the "sometimes" is much more frequent on output taps than on line inputs.

I agree with Al;it has always been my experience to use all available taps and then use the one that sounds the best to your ears.

..just go with what sounds the best..don't worry.