If these speakers are primarily resistive, put a DVM across them and measure the impedance. If they are mis-wired and you get something very low (like 3 ohms or lower), maybe your amp just cannot drive that low of an impedance at the higher SPLs.
This is a simple test and will give you some idea of the issue. Also, compare left with right channels and see how they compare.
If both channels are the same and above 4 ohms, you may have a deffective amp.
However, a simple DVM resistance measurement still will not tell you if you have a capacitive load issue.
The previous poster was correct in that too high of a capacitance load will cause some amplifiers to become unstable and oscillate.
One quick test you can try is to get a large 50-100 Watt 5 ohm power resistor and connect it in series with the output of your amp. If the amp functions OK with this resistor connected (the volume will be 6 dB lower but will still let you perform the test, and the sound quality may not be great but who cares, it's just a test), then you may have a capacitance issue. This test may also give you an indication of whether the speaker loads are too low for your amp.
This is a simple test and will give you some idea of the issue. Also, compare left with right channels and see how they compare.
If both channels are the same and above 4 ohms, you may have a deffective amp.
However, a simple DVM resistance measurement still will not tell you if you have a capacitive load issue.
The previous poster was correct in that too high of a capacitance load will cause some amplifiers to become unstable and oscillate.
One quick test you can try is to get a large 50-100 Watt 5 ohm power resistor and connect it in series with the output of your amp. If the amp functions OK with this resistor connected (the volume will be 6 dB lower but will still let you perform the test, and the sound quality may not be great but who cares, it's just a test), then you may have a capacitance issue. This test may also give you an indication of whether the speaker loads are too low for your amp.