BIAMPING

Half an answer - Using your 125W example, assume a 5 ohm nominal speaker impedance for convenience. If the amp can supply the 125W to the 5 ohm load, it is supplying 5 amps. P=I*2 x R > 125 = I*2 x 5 > 25 = I*2 > I=5. To do this, the output must swing 25 volts. I = E/R > 5 = E/5 > E = 25. Now, bridging essentially puts the two amp outputs in series. Therefore there is a possible swing of 50 volts. E/R = I > 50/5 = 10 amps. P = I*2 x R > P = 10*2 x 5 > P= 100 x 5 > P = 500 watts. Apparently, though, the amps can't quite supply the 10 amps because of component limitations (transformer, transistors, etc), but can only supply about 9 amps, so 9*2 x 5 = 400 watts limit in that configuration. PS - a single one of those amps could also supply 9 amps, but that would be about a half ohm load, which it not where it's rated! :)

It looks to me like you have the following options:

1) Run each amp bridged mono, wired in to each speaker.
2) Run each amp stereo, one for highs, one for lows
3) Run each amp stereo, high and low of one speakers in one amp and v.v.

I went through this scenario fairly recently, and there are things to be set for each of the three setups. I preferred using scenario 2, using one amp for left and right mids, the other for left and right bass. It just seemed to create an energy and bass clarity that the other combinations did not.

(I did this with a set of NHT 2.9 speakers I own, and two ML331's I had on loan.)

Hmmm...perhaps an important point to make here... If the speaker just has dual connectors without some sort of shorting bar you can remove, the speakers could be BI-WIRABLE but NOT BI-AMPABLE. I don't know if there are any speakers to which this would apply, but if you have no shorting bars you can remove, be extremely careful! Tying the outputs of two amps together is almost certain cause of serious amp damage...

More answer - in the example, if you can't play your system loud enough (at least on peaks) then perhaps you are output voltage limited and need the higher voltage swing to drive your speakers (and ears) to their potential. Voltage limitation like current limitation current limitation can result in clipping and distortion or damage. That is the argument for bridging as long as the amps can supply the additional current. You must also know that the speakers can accomodate the extra current/power; that is, they were meant to play louder. If your system plays loud enough, even on peaks, you would just be running the level control lower with the bridged amps and not accomplishing much. Now comes the counter responses, I guess. :)