Something does sound funky with the disparities you are describing so it is really tough to know for sure what's happening but it sounds like it's getting figured out and rectified.
Just thought I'd take a stab at question #1 How can you blow a speaker that is 400 watts with just a 125 watt amp?
A good question and many years ago, I thought exactly like you. Basic Math right? However, I'll take a stab at how the opposite can be true in layman's terms.
I don't know how loud you were trying to reach or what issue was creating the need to drive certain speakers louder than others but in regards to question #1, if you keep requesting more volume , essentially the speakers will attempt to suck more and more power from your amp. Because the speakers are rated higher than your amp, you eventually reach a point where more wattage is needed than the amp can actually dish out. The speakers are not maxxed out, but your amp is.
The amp, will try it's best to a certain point. However eventually it can no longer produce a perfect sign wave of signal. The lows and the highs get cut off. If you were to run the signal through an oscilloscope, it would look something like this.
Speakers HATE signals like this as they are supposed to vibrate smoothly in relation to the signal wave. This effect called is "Clipping" and it can and will damage speakers, at the very least it creates distortion. Often amps have protection circuits built in to shut things down before it gets too severe but it does commonly happen to a point if you drive to much volume.
Short answer:
A big wattage amp can indeed blow low wattage speakers
but
an underpowered amp can also damage even high wattage speakers because it can't handle the job.
Am I saying you need a bigger amp? No. Not unless you enjoy ear damaging volumes or your room is huge. 125 watts is fine for most people. Essentially, if you are playing with the volume button and the sound starts to sound distorted or simply worse instead of better, you have driven your amp into clipping. If you hear clipping but you still want to go louder, then yes you definitely need a bigger amp...... and probably a hearing aid down the road (room size aside).
I have no idea what was the 100% cause and effect of your issues but I thought this bit of trivia would be handy to know in any case.
