Considering the comment and the increase in "help" for you, is this a possible passive-aggressive move by your manager to try to signal to you that your time is limited?The more I think about it I believe this is the correct approach. He's younger, and probably made the comment as he would with the boys after a game of beer-league hockey. But this is a professional relationship, not a locker room. We'll see what happens - thanks guys!
Are there any competitors in your area that would be more appreciative of your skill sets and job knowledge than your current employer is? From the sound of it, you're given a lot of freedom to perform your task because you do what you do well and produce results... at time at great expense to your personal time.
Maybe it's time to move to another company? Or the threat of losing you to another company could make your manager realize you're worth more than the 2 employees he just hired to help?
I'm NOT suggesting you cast about for a new job and use that to leverage a raise with your current employer.