You tend to get a little idle hum in high gain tube amps. Check your cabling, guitar electronics, and power too. Watch for other electronic interference. Your monitor is a great place to pick up noise. Expect some white noise once in a while too from a tube amp. I haven't had any so far with my Mesa's but my Marshalls over the years had a little once in a while. Until I go used to having the fan running I kept looking for where the noise was coming from with my Mark IV.
