Jtinn and Brrgrr, you do not need to use the same amps top and bottom. Many people mix and match when biamping horizontally precisely because they want each amps' distinctive characteristics, tubes on top and solid state on the bottom for example. The amps do need to have the same gain however. If they don't, then whatever is connected to the higher gain amp will sound louder for a given volume setting. So you could have woofers that sound louder than tweeters, or vice versa. It would be like having a vetical balance control within a speaker. If the amps don't have the same gain, some will have an internal setting for gain adjustment. Others would require a technician to adjust one to match the other.