Many of these massage boyz sit in front of the shop and may not even get a single customer that day thus no money, not even 330 Baht. But again, your reasoning, and that of others is simply a justification to give a low tip. Each to his own. But I do agree, over tipping for a poor massage and a not so happy ending , doesn't deserve a generous tip, just a fair tip.
In my opinion a fair tip would be equal to that of fee the house charges, unless the massage was a total flop.