If you expect quality to improve in these spas, the owner should charge more for the massages to usual and customary levels, pay the masseurs a lot more, provide adequate training and feedback. Handouts rarely work. There would be no incentive to improve if you're getting what you want with minimal effort. I've had disinterested masseurs who barely put effort in the massages, mechanically finish the massage, and still expect a 100% tip. Generosity will certainly make that attitude flourish.