classical label monitors

Sony, Warner, Polygram, Naxos, Telarc, EMI, Colombia, Decca
Paramount, Nimbus, Sain, Sentinel, ECM, Virgin, Hyperion etc. use ATC - you can find a partial list here here - it is likely that about one third of your modern classical collection (last 30 years) will have been monitored through ATC's.

I missed Chandos in the list above - like I said - most everything done recently.

Aren't studio monitors not the usual choice for relaxing music listening and for the far more critical needs of the producer & production process?

Absolutely. Horses for courses. The majority of systems for home listening tend to be primarily aesthetic looking for relaxing listening. These designs (and the rooms & the position where they are installed) often fall very far short of the standards required for the critical needs of professionals.

Two-seater sports cars with loud engines and no trunk space and no rear seats are certainly not relaxing either and are impractical for taking kids to soccer practices - which is why we have SUV's and ever so popular Dodge Caravan. A studio monitor will only be appreciated by a very small minority within the already small audiophile minority.

The PMC website has an impressive "key client list".

And PMC used ATC midrange for years and recently their own clone of ATC's three inch mid dome design. So two of the more popular studio monitor designs happen to share the same mid range (and cross over frequencies).