How much to tip the home theater installers?

I am finally having my home theater installed. It's a major project, 3 days. The guys are doing an excellent job. Should I tip and what would be appropriate? I was thinking $40 each for the two 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.