Apogee vs Soundlab

Dazzdax, its normal for transistor amps to sound that way on the Sound Labs, due to the low impedance at high frequencies. Sound Labs do not expect the amp to double power as the impedance is halved. see
http://www.atma-sphere.com/papers/paradigm_paper2.html

So when you put a transistor amplifier on the Sound Labs, the result is too much treble energy and not nearly enough bass. That's why tube amplifiers are preferred; they will try to make the same power regardless of the impedance- that gives you a more tonally neutral sound on the Sound Lab.

Bottom line is that they can be very extended and smooth yet still aggressive if the music calls for it, IOW quite neutral :)

Dazzdax, one thing you might try is to put a resistance in series with the speaker. Two to four ohms ought to do (some audition might be a good idea), but it should handle high power. This will simulate an amplifier with a higher output impedance and will substantially reduce the power doubling problem. A fair amount of power made by the amp will be absorbed by the resistor, so this is not a very efficient means!

The impedance curve of the Quads is bit different from the Sound Labs, if I recall right the Quad matching transformer operates full range (the Sound Lab is a full-range diaphram driven by two transformers which are crossed-over to operate two frequency bands). I doubt that it has the same bandwidth as a result.