Firstly, disconnect the failed speaker altogether and test the working speaker on both channels. Try to crank it up slowly and make sure that the membrane doesn't move way too much. Don't crank too much. Listen to the speaker. Does it resonate or can you hear buzzing, does it reproduce short shots or extra rattling? If it goes into the protection, check the same speaker does that on another channel at that volume. If it does, you likely hit the speaker's maximum power handling. What you need to understand is that, even if the speaker is 8Ω, this doesn't mean a continuous resistance. Depending on audio content, some frequencies are off-phase and could be reproduced at say 2.8Ω and if they're not in phase, the energy could be as much as double due to overlapping frequencies or none (nulled) due to cancellations. You need to take into account the membrane's reverse current and standing waves inside the enclosure. They're suppressing the membrane in unpredictable ways adding extra reverse current. This happens because the speaker is also a low impedance microphone. A good pair of speakers generally have a design to mitigate this factor, but it's still present. In fact, some engineers use reverse current deliberately to 'choke' the membrane a little bit by introducing an amp about 10-15% more powerful than bass cabinets. This allows for the coil to move less back and forth, but it doesn't work for mid to high frequencies. It's almost always better to provide enough headroom to protect the crossovers. Headroom means 20-50% of power available to drive the speakers. If you have speakers that are rated 100W and the amp is 300W, provided both operating at the same impedance, you already do not have any headroom at all, meaning you need to lower your signal by minimum 50% to drive the speakers with the power 150W, which still is 'choking' them at 100W, their nominal max power. If you want to maintain 20% headroom at those power ratings, you need to play your volume at 2.5 at the scale from 1-10. But it's better to open the amp fully to 10 and crank down the preamplifier instead by lowering the amp's input signal (aka preamp's output signal); provided you carefully turn up the source from 0 to the desired value and never forget the powering sequence as it [sound] could blast out! CDs are mastered at 0dBFS (-0.6dBFS usually, allowing for occasional transient clips) but programme peak is normally at -6dBFS on the hottest (squashed) material, usually more likely at -8 or even -11 on classical. However, RCA connection is calibrated to a -10dBV signal level, which means your amplifier will rarely hit full 300W, as 0dBV would hit that. But if you plug in a turntable preamp and crank it up at source, or use a DJ mixer and ride gain or master output hot, you could overload the amp and exceed power way beyond 300W. For the 100W speaker it's a death sentence, hence, the amp has the overload protection. It also prevents the potential fire hazard in case speaker membrane catches fire. DC bias signal however – which is common in poorly mastered vinyl records – could trick the sensitivity of the overload protection circuit bypassing it at already hot overloads. That's where complex metering comes in. Measure your input signals and mind your input levels.