A few weeks ago I mixed a band I did not know, at a venue I had never worked. The band was great and the show went well, but I fought the system all night. During sound check the amplifiers were clipping. I had already walked the gain structure from the main bus through the matrix feeds to left, right, and subs, so the console was not the problem. The amplifiers did not have the power for that room or that music.
After the show I asked the owner who designed the system. He said they read the wattage on the speakers and bought amplifiers rated below it. The next day, I spent a couple of hours with him going over what an amplifier spec sheet tells you and what it leaves out.
A loudspeaker's power rating describes how much the box can survive over time. Sure, it sets a ceiling, but it says nothing about what the room needs. Sizing starts with the level required at the farthest seat, the speaker's sensitivity, and the distance. Then comes the program. A live band can put peaks 15 dB or more above its average level, and the amplifier has to pass them without running out of voltage. Compressed background music needs far less. Duty cycle changes the answer again, because hours near full output run into heat and power supply limits.
The 2x-the-speaker rule is a conversion between rating methods that assumes one kind of program. Sized that way, an amplifier can cook a voice coil under a dense, limited mix for 4 hours and still clip on a dynamic band.
One argument for bigger amplifiers gets half a concession from me. A clipping amplifier raises average power and pushes extra energy into the high-frequency driver. What protects the driver is a limiter set to its ratings, and that is true at any amplifier size.
The biggest number on most amplifier sheets is the least useful one. Peak, dynamic, and burst figures describe what the amplifier can deliver for a fraction of a second, and a set runs for hours. Instead, find the continuous power per channel with all channels driven, into the load you will connect, across the full audio band, at a stated distortion figure. On a multichannel amplifier with a shared supply, the headline per-channel figure often assumes some channels are resting. Then size it against the program the room will actually play.
Reading the spec sheet and assuming that 800 watts is going to be just fine is not giving you nearly enough info to specify an amp.