IF the '685s follow normal B&W practice, they are a real handful in the lowest couple octaves and may even dip to 3 ohms with a largish phase angle. This is amp killer territory. With a crossover at 4khz, you could do with an amp of 1/4th the 'wattage' above crossover than below....easily, and never clip the HF amp.
http://sound.westhost.com/bi-amp.htm
It wouldn't surprise me even a little that the combination of being able to turn it up, slightly, and the reduction in HF distortion allowed a higher playback level with the same perceived loudness as 'before', while the bass balance shifted lower. Same SPL to within a few db. Even on my system with LOTS of power, the bass opens up when I play at a radioshack analogue meter'd 80db. I believe ear sensitivity plays a large part in this...at lower levels, especially.
The impedance curve of the speaker? almost irrelevant when dealing with a SS amp. Note from the Stereophile test of the Harbeth you link that potential probems with rising response were with a Tube amp...not SS. Stereophile is generally more concerned with wacky phase data when it occurs WITH impedance minima.
http://sound.westhost.com/bi-amp.htm
It wouldn't surprise me even a little that the combination of being able to turn it up, slightly, and the reduction in HF distortion allowed a higher playback level with the same perceived loudness as 'before', while the bass balance shifted lower. Same SPL to within a few db. Even on my system with LOTS of power, the bass opens up when I play at a radioshack analogue meter'd 80db. I believe ear sensitivity plays a large part in this...at lower levels, especially.
The impedance curve of the speaker? almost irrelevant when dealing with a SS amp. Note from the Stereophile test of the Harbeth you link that potential probems with rising response were with a Tube amp...not SS. Stereophile is generally more concerned with wacky phase data when it occurs WITH impedance minima.