I've tried a carvin bx1200, running only one side, tried different crossover frequencies
tried a hartke 3500
and tried a carvin probass 300
using the same instrument, and same amp settings, I can get great sound without distortion at about twice the volume with a 15 year old 4 ohm carvin 210 that spent a couple months outside (thought it was stored in my fathers house, found out he put it behind his garage :[), the cones on the carvin are covered in elmers glue just to hold them together after they dry rotted. Between 2 and 3 on the hartke with the 4 ohm carvin, the speakers will start to buzz a little. Between 1 and 2 on the same amp, with an 8 ohm avatar 210, it sounds like the speakers are "farting"