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.
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.