It very, VERY much depends upon the amp (early Acoustic Image or other downward-firing woofer design? Port location? Other drivers?), the room acoustics, the location of the amp (corner? Floor/wall junction? Proximity to wall(s)?), overall volume required, pickup used, genre of music....
No single answer will work. My Acoustic Image Coda sits on the floor, but I only like it for very quiet small group gigs. Genzler Acoustic Array Pro sits on a portable table about 20" off the floor for a nice, clean, clear sound audible evenly everywhere, and still enough bottom. For big band and large rooms, instead of the table, I put it on top of a BA10-2 cab for extra "feel" down low without having to play a lot louder.