One of the things I've done to deal with underarm stink build-up is periodically wash my shirts in sports wash. There's special detergent that you can get for tech fabric/sports gear that works when on non-sports gear too. It's a little expensive, but it's not something you have to use every time you wash your shirts, just when you notice the odor building up. And because it's meant for tech fabric, it's kind to most fabrics - I've used on two velvet shirts that like to hoard scent and had great success. You can get tech wash at outdoor gear shops.