Never cover vents on electronic equipment when that equipment is on. Heat is the enemy of electronics (even tubes). Covering vents on operational equipment is asking for failure!
heat
i only use my stereo on sundays for 6 hours. i recently started to put paper towels above the air vents on my pre amp tuner and power amp to keep the dust out. i have a mcintosh mx132 and mc602. is this a bad idea? i know heat is the worst enemy for electronics but i don't play it loud anyway!