I hear that alot also. Get rid of bio balls and get the live rock. Biff had a good explanation as too why bio balls dont do the same as live rock. I think it had something to do with the nitrate and nitrite. If i can find it ill post it.
I'm actually doing that exact thing tonight. Its definitely a good way to go and it also means you have a fuge If you don't already have one instead of crappy bio balls. I have put it off for over a year but I'm doing it tonight.
My research I've been doing today says to use rubble, 1"-3" cured. Any more input would be appreciated. Any trouble with detritus collecting in the sump? Thanks all for responding.