My 2 years comparing audio equipment: Audiofool placebo, marginal gains, law of diminishing returns

My frame of reference. Equipment that I have tried/owned.

IEMs:
Thieaudio Prestige Ltd £1149
Campfire Audio Dorado £1099
Elise Audio X Helios Symphonium SE £1099
Thieaudio Monarch Mk III £899
Thieaudio Monarch Mk II £800
Sennheiser IE600 £600
Thieaudio Hype 4 £350
Fiio FD5 £295
Ziigaat Cincotres £255
Ziigaat Doscinco £255
Penon Serial £247
Fiio FH3 £99-120
Fiio JD7 £89
KZ ZS10 Pro £45

DAPs:
Hiby R6 £1400
Sony NW-WM1AM2 (Uncapped) £1300
Fiio M23 £649
Hiby R6 Pro 2 £600
Hiby R5 Gen 2 £450
Sony A306 £329
Sony A55 £200
Hiby M300 £200
LG-V60 £200
Hiby R3 Gen 2 £179

In my experience, with the specific equipment listed (IEMs/DAPs), I have not heard the difference or scale of difference in certain equipment as often claimed by "audiophiles". I have never seen any "audiophile" making claims to have any objective evidence. Something like a proper blind A/B test demonstrating they can consistently tell differences between X/Y/Z equipment.

I have encountered what I call the price of entry barrier fallacy. This is when an "audiophile" claims that the reason I cannot hear "significant difference" between equipment is because certain equipment has to cost X amount to be capable of revealing a difference. For DAPs I was told that I won't hear differences between specific £200-600 devices and need to compare to a £4000 Cayin DAP. Most recently I was told that "tranducers" need to cost over £800 (Thieaudio Monarch Mk III & Prestige Ltd) to hear "significant difference" between the LG-V60 and Fiio M23. I have no idea if the Monarch Mk III, Prestige Ltd, Hype 4 use the same or different transducers.

In a single listening/testing session I have tried the Thieaudio Prestige Ltd, Monarch Mk III on the Sony A55, Hiby R5 Gen 2 & Hiby R6 and heard no "significant difference" between those DAPs. I don't think I could tell the difference in a blind A/B either. The £1400 Hiby R6 perhaps sounded fractionally cleaner, maybe 1 or 2% but that is likely just placebo. The 3 DAPs probably did not sound 100% identical but the difference between them was barely perceptible/marginal. Perhaps 1 or 2%.

With the Hiby R5 Gen 2 I heard multiple "audiophile" reviews claim that the "Class A" Amp mode significantly/audibly improved/changed the sound. When I tried it I heard no difference at all, let alone improvement. The only caveat to add is that obviously my testing was done on my specific IEMs at that time which would have been somewhat different to those of the "audiophiles".

The largest difference/jump in quality that I have experienced in the past 2/3 years was when I heard the KZ ZS10 Pro after using the AKG EO-IG955 & Samsung EO-EG920BW with my Samsung Note 9. The KZ ZS10 Pros were significantly better. What I would call a "night and day" difference.

Price barrier of entry fallacy:    • Thou shalt not blaspheme | Price barr...