The #6 and #8 screw extractors are the easiest way, by far, to remove those stubborn Symmons shower-valve hot and cold seats! A handy-man is best to complete this job. I'll explain the procedure the best I can just incase you decide the job might be for you. You could probably purchase the screw extractors at a plumbing supply store in your city or town. Call them ahead of time to see if they're in stock and how much they cost.
The only thing is this: You'll most likely use these screw extractors once in your entire lifetime as they are not common tools of any household handyperson or handyman. At least they are not too expensive to buy and are VERY simple to use, but a little "muscle-power" might come in handy.
If you can, it might be a good thing to also bring the Symmons valve seat removing tools back to the hardware store to get a refund!! You wouldn't be the first person to tell them that those Symmons tools are worthless except at stripping brass!
O.K., now for the task at hand. Here's how to complete the work:
You'll need:
#6 screw extractor
#8 screw extractor
An adjustable wrench (found at any hardware store) that fits the square-end of both screw extractors.
A large pair of adjustable pliers (just in case) to help hold the shower valve, itself (VERY IMPORTANT) from turning if the seats ARE REALLY TIGHT and you need to apply some pressure.
Fit the screw extractor (either #6 or #8 depending on which seat you need to remove) directly into the shower valve seat leaving the square end hanging out towards you. Fit an adjustable wrench onto the square end and begin turning the wrench counter-clockwise while pressing your thumb and on the end of the screw extractor (the end that's nearest to you and your adjustable wrench). The extractor will grab the cold seat (or the hot seat) just like using a screwdriver to remove a tightly fit screw. If it doesn't begin to turn or come off, you'll have to press a bit harder with your thumb and might even need to use a small hammer to VERY, VERY GENTLY tap on the end of the adjustable wrench. Make sure you are especially careful not to put stress or begin turning the entire shower valve as this could damage the water pipe soldering and could cause water leaks afterwards!! Be careful, go slow, take your time, don't rush! Check the valve for leaks after the assembly is put back together and the water is turned back on. If all is good all you have left to do is to place the chrome face-plate and handle assembly properly back together onto the valve!
After all is said and done, you might want to put those large screw extractors on eBay to get part of your money-back.