Harbeth had a long term offer of free speakers iirc for anybody who could tell the difference between 2 properly engineered and suitable amplifiers.
What, they (Harbeth) picked two amps of their choice?
They would not allow some other suggestions for the tested amps, then evaluate whether both amps are properly engineered to allow a contest?
I say this because you can indeed find two amps based on the same topology, similar power and specs, which will indeed sound similar even in a non-blind, non-level matched test to any audiophile who is not being heavily biased. No biggie for them to make a contest out of such. Should it prove anything? It doesn't imply you can't find another two properly engineered amps of different topology sounding different enough to prove it in a DBT.
Funny, if I wanted to prove the difference can be heard in a DBT, I'd pick two amps and my own music samples. Then others are free to evaluate whether those are both properly engineered, with sufficiently flat curves and without notorious distortion etc. Then we agree on DBT listening volumes so objectivists can confirm there should be no clipping. Then mutually agree methodology of DBT. If I wanted to prove anything I would like to have my own terms, with the other side reviewing terms and methodology and agreeing whether all is correct. Not to have DBT in someone else's terms, which can be manipulated so they won't easily show any difference (bad choice of samples for the discerning purpose, for the example).