"I've heard varying answers to this question so I'm coming to the experts. I run a small PA for my duo. The main amp we use is rated at 200 watts per channel at 8 ohms, 400 watts per channel at 4 ohms, and 400 watts per channel at 4 ohms mono bridged. It seems to me that no matter how I connect my two 8 ohm speakers up they are going to be getting the same amount of power and sound the same, yet I've been told by various 'experts' that this way or that is better. What should I do?"

There are three obvious methods that you appear to have identified to connect the two speakers to your amp. 1) Each speaker connected to one side of the amp normally. 2) Both speakers connected to one side of the amp. 3) Both speakers connected to the amp in a mono-bridged configuration. (For 2 and 3 we assume the speakers would be connected in parallel with each other.)

Clearly in terms of rated power delivered to each speaker each of these three most obvious methods are equal. This is unusual, as I have never seen an amp where these figures worked out to be so even. And the "reality" of what's really happening likely isn't this even, but we'll go with those figures for now. Different types of power amps may react differently to different types of loads so there's no single method that's always best. Running both speakers off of one side of the amp does free up an amp channel for other things without a significant penalty in performance, but if it isn't necessary I wouldn't take this approach. Often times sound quality and real world power output can suffer to some degree. It depends a lot on the amp. Comparing the two-channel scenario to the mono-bridged scenario is even harder to sort out.

When you mono bridge an amp you run the two channels at opposite polarity, but tie them across one load. This means the voltage swing is twice what it normally would be. When one terminal is at its most positive voltage the other is at its most negative. So you get a lot more power, but it's from a voltage increase. There isn't necessarily going to be more current available. Driving speakers requires a lot of current, especially where low frequencies are involved. Therefore you may or may not benefit from having it connected one way or the other. You pretty much just have to try it and see if you can tell any difference between the two.

A fourth, less obvious, method of connecting the speakers exists, and it's very likely it could provide the best performance of all. Switch your amp to mono bridge mode, but connect one speaker to each side as you normally would, only reverse the polarity of one side. By switching the amp to mono-bridged operation the polarity of one of the amp channels is reversed. Reverse the polarity of that speaker connection and the two speakers are working in phase, but one channel of the amp is backwards. Why will this improve performance?

When you observe the impact that music has on an amp you'll notice that most of the draw comes on kick and snare hits - with popular music this is often beats two and four of each measure. The way the power supply of most amps work is analogous to a reservoir of energy. The power supply works to fill it up and the amp then uses that power. There is a positive and negative reservoir and these are filled periodically at a rate of 60 Hz, the AC line frequency in the U.S. When the signal is in the positive half of a waveform the amp draws power from the positive reservoir. When in the negative half the negative reservoir is used. Not all amps work this way, but most "conventional" amps do. The result is that when the two channels are amplifying an identical signal their use of the reservoirs is the same. At times of peak operation the positive side gets depleted, and then the negative side, though each cycle of the audio waveform. But when you operate one channel in the opposite polarity, as described above, each channel effectively gets its own reservoir, which can give you a little bit more headroom, especially on low frequency sounds.

Keep in mind, however, that all of these variables are likely very minor in your case. Remember that doubling the power is only going to give you a 3 dB increase in SPL, best case, and this isn't going to give you anything anywhere near that. A sound company operating dozens of amps has to think in terms of percentages of improvement like this as it could result in one fewer rack of amps and speakers needed on a gig, but for your duo it isn't likely to make a profound difference.