For about three years now we have been running the reference speaker system that we designed right before VSAC 2008 with a lot fewer amps than it was intended to use. Finally, today we got back up to four Paramounts per 90dB sensitive 4 ohm midrange driver, each set up for 1 ohm output and all four secondaries wired in series. Since I have had so much luck with a balanced output improving the bass and lower midrange of anything we set up that way (including the amps on the system's woofers), we floated the ends of the series wired secondaries and tied the "center tap" - the junction between the two middle secondaries, to safety ground. Mids cross to woofs at about 250Hz.
We also completed a small mod to our SR45 tweeter amps, adding a cap at the input to further reduce signal from the preamp below 500Hz (tweeters come in about 2kHz), to eliminate some distortion we were getting from the tweeters when running the system hard.
Doing measurements in a logical sequence, we saw no negative effect from the new tweeter amp filter when we measured the system and it definitely cleaned up the problem we had. So on we went to putting the midrange amps back the way they were supposed to be - four on a side, secondaries in series. Then we dialed in the overall frequency balance by shooting each speaker with the mic set at the listening position. Though the frequency response measurements looked similar to the previous setup with only a pair of Paramounts wired in parallel on each midrange, the sonic improvement was of "holy shit!" proportions. The lower midrange and upper bass have an ease and clarity that I been missing for quite a while.
Suffice it to say that it was enough of an improvement that I spent about an hour listening to favorite tracks before leaving the shop this evening. If you haven't heard the system in a while, I think you will be surprised next time you hear it. It looks a lot like it has for a while, but darned near everything has been refined in the past year - room, speakers, amp configuration, preamp, DAC, etc.
