Shouldn't amp reviews employ multiple speakers?

I would agree somewhat. Using one set of speakers that you are familiar with will enable you to discern differences in amplifiers more readily. Especially if the speakers have a quality of being revealing - such as the Thiels - and are considered to be at the upper end of the spectrum.

A reviewer is trying to give a reading on only the amplifier; to attempt a comparison to other amplifiers (FWIW). Whether an amp is of lower or higher quality, it's characteristics will be evident thru a good pair of speakers - then you can assess the reviewer's comments and the amp's measurements as to how it may pertain to your system. After all, if you buy the amp, the final determination as to whether to keep it will be on the basis of how it sounds in your system and not the reviewer's comments (hopefully).

I would also say that if an amp "sounds terrible" with other speakers: is it the fault of the amp or the speakers?

That's part of the appeal of this forum. We can share our experiences with those who have similar gear. Furthermore we can get a consensus.

Gs5556 makes some good points. I have one of my systems set up to be as revealing as possible for testing electronics. You really need a linear, low-distortion, wide-range system to hear what electronic components are doing (or not doing). I mean, you can't comment on dynamic contrasts if your system is dynamically constricted, and you can't comment on bass quality or treble quality if your system does not have adequate resolution at the frequency extremes, and so forth.

Lots of folks don't even know what their amps really sound like because there are so many other limiting factors in their chain of components...

Amp designers had to play their amps on at least one set of speakers. Good ones will use several. I wish they would disclose which speakers they use or have found to bring out the best in their designs. Years ago Magnepan and Audio Research did this. Anyone know of any modern unions?

Well they'd have to try the amp with several speakers, different room sizes, different preamps, different interconnects, different recordings, different racks they sit on, etc.

At some point you just have to take reviews as guidance. That's why you should listen for yourself before you buy.

As my system evolved, I noticed that my equipment may have different synergy but still seems to have the same characteristics no matter what you match it with.