As far as I know and in my experience, Ernie Ball MM have stock of spare parts available only if you (or an authorized service) send them the old broken part. They will stick with the original type of part that the instrument left the factory.I’m kinda late to this particular part of the conversation, but with regard to the availability of that preamp, I replaced the preamp in a 1979 pre-EB (of course) Stingray. I called MM and inquired about a replacement for that particular model and the reply was, “sure, no problem, just send us the bad preamp and we’ll get a new one right out. That’ll be $65, please”. And I was warned that it wouldn’t look like the bad unit I was replacing, but it was the exact replacement. And they were right. It definitely had that early Stingray tone.
I did ask and the fellow told me they had a good stock of pretty much all the old-model preamps available with the exchange of the bad unit it was replacing.
I was mentioning this because I was surprised that you were told the preamp you needed was unavailable. Perhaps things have changed.
Asa side note, I know of three older stingray owners that swear the John East MM replacement is the best aftermarket Stingray preamp they’ve come across. If I were in need of a new preamp and I couldn’t get it from MM, I’d go with the East unit.
What is not gonna happen through them is changing any part for a different one, for exemple, replacing steel frets for aluminium ones or the other way.
What is really surprising is that they have the pre-EB parts in stock.