I just got finished upgrading my M3s toM3turboS, and though Clayton said the differences would be "incremental", I found them to be very large indeed. After I had the M3s a month, I loved them but felt the upper end could use a little help, so I had them converted to m3turboS. WOW!! I dont know all the terms, but I will say these are by far the best speakers I have owned, which included big Maggies and B+Ws. I drive them with a 35wpc tube amp, and it is a combo made in heaven. As far as being efficient, I replaced montior audio speakers, supposed to be very effecient, but the M3s get much louder with very little amp power. They sound excellent at all volumes, and their waf is off the wall. I am 66 years old, and for the first time ever I feel I will never need another speaker system( or amplifier) Don't walk, run and get these! at $2600, they are just stupid good. Thanks Clayton, for making this type sound available to shleps like me!