The A+B terminals are intended to allow two pairs of speakers to be connected. You should use speakers rated similarly on both sets of terminals if you are going to use them in tandem. It is unlikely that any damage to the 2150 would occur if you did but, the speakers will respond differently to the amount of power supplied to the outputs. You will get an overall output impedance less than 4 ohms overall. You could probably trim a set that was responding hot but it seems to be the counterproductive thing to do. I would look for another pair of speakers that had a similar impedance and SPL rating. Depending on how well matched the pairs of speakers are and their relative strengths you can get signifivant performance improvement with the second pair online.