@lewm I know all about discards due to testing matching, I supplied a substantial amount of monies for the Parts used on the Two Valve Phon's Prototypes of which I have one which incurred further costs by having the End Design voiced to my own preference.
I do believe the individual you have quoted does produce a RIAA that falls into the tight tolerances quoted, by being in a room with the builder, I vaguely remember their description making a reference to the RIAA and on other posts on forums they certainly make reference to it.
On the day a Two Box Paradise 0.02 dB RIAA? and a Four Box 0.3 dB RIAA?
The RIAA dB's quoted are very likely to have been these, unless the Four Box Builder selects a tighter tolerance for their own Phon's RIAA, than the one they reference as being the purchasers selection,
On the day, as made known in earlier Posts about the Paradise, the Four Box Model in my view, was the Phon' worthy to be Standalone A/B Compared to the resident £10K Phon' and separating them for which was the better was no easy task.
As said, this in my view, places the Paradise in a position where for the monies to be laid out to achieve one, gets an individual a Phon' will comfortably hold its own with Phon's from the Branded World that retail at £10K and upwards.
What's not too like if one is a Vendor, Probably a Fly in the Ointment if one is a Retailer.