Beetlemania may have directly compared coils to coils
I did not make this comparison as I replaced all caps and coils at the same time. Not possible, with so many simultaneous changes, to isolate the sonic differences from coils. I spent close to four months elapsed time making changes, allowing burn-in, and making sonic comparisons in mono with the modified and “reference” channel.
1) OEM FST boards (sandcast resistors) to FST with Mills MRAs
2) FST with Mills to Masonite boards with Clarity CSA throughout, Mills, and “Lex” or Erse foil coils, the latter on feed positions (ie, everything different except Mills)
3) New boards with and without Multicap bypasses on coax feeds
4) 160 versus 250V caps on woofer shunts (I heard a difference)
5) OEM hookup wire and binding posts versus Cardas wire and binding posts
6) Single versus biwire from amp
I would not lose sleep over unavailability of six nines coils. If you have these in your classic Thiels, great! If you have FST, see if Rob Gillum has “Lex quality” or buy the best you can. I mean, we’re talking about speakers 10-30 years old that were modestly priced to begin with. Sonic improvements from better caps and resistors are notable even if I can’t really speak specifically to coils. That said, IMO, it’s worth putting foil coils in the feed position as these are not expensive.