OK, then get an amp with enough power where there's a 100% chance it won't ever wimp out on you.
Determine the loudest level you want the music to be and how far from the speakers you want this peak level to be at. Then you can calculate how many watts will be required to reach the target SPL at the given distance based on the speakers sensitivity with plenty of headroom to spare so the amp never distorts let alone clips.
For example, you want to hit peaks of 105dB at four meters (12ft) from a speaker with a sensitivity of 94dB into 8 ohms:
This means the speaker will put out 94dB at a distance of one meter with one watt of power. As I understand it, adding a second speaker will increase the output by 6dB at one meter. So with both speakers playing, one watt will give an SPL of 100dB.
Next, for every doubling of the distance, the SPL drops by 6dB. This means that at two meters from the speakers the SPL will be 94 and at the target distance of four meters the SPL will be 88dB.
Next is to determine the amount of power needed to reach the target of 105dB. To do this you need to increase the volume by 17dB. For every 3dB of increase the power needs to double.
1W = 88dB
2W = 91dB
4W = 94dB
8W = 97dB
16W = 100dB
32W = 103dB
64W = 106dB
Next you need to allow for enough headroom to keep the amp in it's zone of linearity (the area where the distortion of the amp is relatively flat). With tube amps, the distortion curve stays relatively flat through the first half of it's power output then increases sharply in the second half. The zone of linearity for a solid state amp is roughly 10% -20% of it's power output.
So to reach 106dB at four meters, a tube amp would need at least 128 watts and a solid state amp 320 - 640 watts depending on the distortion curve of the amp.
