It's a curious article, based on pics alone because the text is very hard to read. ...
I squinted pretty good to read the article, but could only reliably make out the inset "Why Was Clapton's Amplifier So Different?" and text on that page.
It smelled of "fairy-shit" to me (a mysterious combination of fairy dust & bullshit). There were several things which start at a conclusion (how the amp sounded on record) and asserted pseudo-tech reasons that lead to the conclusions... except the pseudo-tech is described to do things in ways which don't really happen.
The bit about KT66's "loading the driver" is completely bogus as a cause for distortion in the driver/phase inverter. I may be mistaken, but I thought EL34's came along later than the amp discussed, so why do they muddy the waters talking about them? It is true the KT66 requires more drive signal than EL34s for the same output (because the EL34 has higher Gm), but the 5881 has even lower Gm than KT66s and is therefore even "harder to drive"... Which means what exactly?
I also saw reference on the same page of "trick-matching the primary" which is only a "trick" to folks who don't know how transformers work. It's claimed to make the OT or output tubes sound different... More fairy-shit.