I just remembered one last thing as I sat listening to some Zappa instrumental material: as you go up the ladder with their models you get increased resolution. You'll hear details on the 1.7s which weren't apparent on the MMG/12s, you'll hear additional material on the 3.7s and there is a jump up with the 20.7s as well.
That's what the money buys along with increased bass, larger soundstage and the ability to play easily at high volumes.