Home "Hi-Fi" magazine's reviewing processes are not a very good barometer of what gear you should choose professional touring gear Viabcroce. And your premise doesn't support, IMHO, your conclusion that any given 'higher specified external amp' will result in this: "chances are the rig will sound better". Assuming I am misunderstanding you, can you define 'much higher specified' and 'better' within this context? I'd like to understand what you are driving at.
Sure
My praise about certain test modes in Hi-Fi is related to the method and the approach.
It has been proven that, no matter how good an amp is, it might not result the best choice for a given cab. As for what I consider, in general, a "good" amp, this relates to its ability to linearly output voltage, keeping the distortion value low, at the different frequencies. While it sounds reasonable that a 3,000 $ poweramp might be "better" than a 150 $ one, this ability is strictly related to the load the amp sees, IOW to the (system of) loudspeaker(s) it's driving.
The distortion generated by the several causes of non-linearity in the power amps is traditionally measured (in the Hi-Fi world) loading the amp with purely resistive loads of 4, 8 and 16 ohm. Due to the combination of a number of issues, the amp behaviour (in terms of distortion and maximum voltage output) when connected to loudspeaker may differ substantially from what measured with the resistive load, and more specifically it changes depending on the frequency. Furthermore, these values are different depending on the nature of the applied load, that is of the loudspeaker system the amp is loaded with.
The point is, that different amps exhibit different behaviours with the same cab. When testing several couplings, you find that some amps are relatively "indifferent" to the cab you load them with, while others are much more sensitive, and their maximum undistorted output varies a lot with the different cabs.
You'll easily see that, while these methods have been developed in the Hi-Fi realm, they are definitely correct when it comes to determine the optimal coupling between a poweramp and a loudspeaker for playing music as well.
The point in my previous post is, that while a "better" amp might be generally considered a better choice for a good sound, a correct coupling may return a better overall result than what you might get just choosing a more expensive amp.