How much to tip the home theater installers?

Never tip. Tips are taxable. So intending to help the person you help the government. What you do instead is gift. Gifts under $10k are not taxable. So make it cash, under $10k, and between you and them and its nobody's business.

When I was young I did some high end installations    The best tips I ever got was the used gea.  Some people would ask if I wanted it and as a college kid I never said no to gear I could use or sell cheap to friends.     If you don’t have any gear to give them 40 bucks each is something them will appreciate