Well, I have always run my tube amps without their metal lids/covers. The result is an opening up of the sound. Almost as if a metal veil has been lifted. Try it.Could be your just exciting tube microphonics and adding false euphonics.
noromance
A simple good test to see if any tubes are microphonic in tube preamps or amps, is to turn the volume up to normal, have someone listen close at each speaker, and for you to gently flick each tube with your finger nail. The person listening at the speaker will tell you if they can hear that flick, bad ones will really stand out.
Cheers George