Critical oppositions made using inadequate equipment.

Not if you are a legit reviewer.

...mho, 'oppositions' is actually a good faux paus...*s*

On changing item X with Z, one ought to either notice a 'change' of some sort.  But it becomes a matter of taste and/or deliberation as to whether the change is good/bad/indifferent is all else is not modified in any fashion.

If there is a weakness already extant, it likely would perform more or less the same....no?

+1 @hilde45 - not enough info to draw conclusions

@surfmuz

critically estimate tt or cartridge characteristics

What does “critically” mean? Hearing any changes? If you mean like a professional reviewer then the entire audio chain must be of high enough quality to hear differences. If the later then the answer is no.

cheap monitor speakers

Also vague. Price is not the same as performance.

In high-end audio, the entire audio chain matters. Often addressing/upgrading the weak link components yields the best sonic upticks.

In order to sonically evaluate an audio component, the rest of the audio chain must be of a high enough quality to hear the differences.  Not enough data to draw any conclusions 

As a matter of principle, it seems to me counter-productive to seek to evaluate a component using other components of a poorer sound quality than the one under evaluation.

But it is more counter-productive to seek to allocate reliable or universal relative sound quality ratings to diverse components.

So the answer is not 'no', but 'the question is not susceptible to a reliable answer'.

There are many cheap mass market products on the market, for example an entry level turntables or speakers, but when it comes to analog playback you can hear the difference upgrading just a cartridge (read above).

Of course if you will start from interconnect cables using such entry level system the difference will be hardly noticeable (if any).