I always tip if I have received exceptional service, regardless of whether is a provider associated with a spa or self employed. For massage I never expect more than a massage and tip according to how I feel the massage was. If the massage veers into escort territory I tip accordingly. And if a provider provides exceptional service, again I tip accordingly. I assume independent providers don't see clients for 8 hours a day. I have a friend who works in a spa as a masseur and none of the therapists massage for 8 hours a day. Physically its hard on them. I also assume that providers keep notes on clients, like I do on providers, and I personally don't mind being classified as "good tipper". I've tipped 100% to one of my regulars who hasn't gotten back to folks on this forum, but I've never had that problem with him.
as @AtticusBK said I don't understand relating masseurs/escorts to lawyers/doctors. It's not the same. In my field I pay partner attorneys over $1600 / hr who give exceptional advice, but at that hourly rate it should be. And I don't tip them. I don't know of any providers who charge that hourly rate.