If I get a well made high priced external phono pre-amp, am I'm in for some revelation?
Phono pre-amps are interacting devices. There is always an interaction between arm and cartridge. Not all of them are a good match. Then there is a substantial interaction between cartridge and pre-amp input and even the cable used is important.
Then there is the actual performance of pre-amp itself.
The latter is what Amir tests. Noise, noise spectrum, gain, distortion, dynamic range, headroom, frequency response, deviation from the RIAA standard.
These are all important by itself as there are many different cartridges that have varying 'needs' to perform optimally.
Here is the thing. When Amir measures Phono pre-amps it is driven by a 'standard' and anti-RIAA source that is not inductive.
This makes measurements comparable but may very well deviate from reality.
Not so much in distortion and noise but it can vary a LOT in FR because there is an interaction between cartridge (inductance, resistance) and capacitance (cable, arm-wiring) and for low frequencies even the arm combined with cantilever suspension.
This means that 2 phono pre-amps may measure equally flat under test conditions (neutral after RIAA) but may well be quite different from the same actual cartridge. There can be peaking or roll-off in the audible range which is not seen in tests.
For MM cartridges the input resistance is usually 47k but sometimes a higher resistance or combination with a capacitance can be more optimal for some cartridges. You could also very well have a 'tuned circuit' at a certain radio frequency (inductor, arm wiring as antenna, capacitance) will create a tuned circuit and semiconductors in the amp can become an 'AM-detector'.
To make the answer short: You could be in for a revelation when you happen to have a cartridge that is optimally loaded or loaded in a specific way that you may prefer but does not have to be correct.
Selecting the right cartridge, setting it up as perfect as possible, loading it correctly and have low noise, low distortion, plenty of headroom and outputting enough voltage for today's 'standard' 2V inputs may well give more revelation than simply changing the pre-amp or its settings.
So aside from the tonal balance (FR) of the cartridge given at a specified load the actual FR may be different on different preamps, arms, cabling and this is very measurable (using a record as source).
In the old days most MM inputs (and MC inputs) all were quite similar and it was universally understood that cartridges (needle, cantilever, magnet, coil) determined the sound. And it does.
Higher end pre-amps can be configured to have better 'synergy' with various cartridges but as there is a difference between cartridges that isn't there under 'lab test conditions' the FR (tonal balance) may differ from what is measured.
In the end we all know vinyl is not more accurate and vastly inferior when it concerns capturing and reproducing audio signals (music) but the 'shortcomings' of this system and 'tuning' as well as perceived SQ of vinyl may well attract a lot of people. They care less about 'accuracy' but more about 'presentation' and their personal preferences concerning sound.