For your next tube roll, try using a pair of RCA Cleartops in the gain stage. These tubes have incredible air and openness, but are completely neutral, so use a warm tube as a follower. They are also very affordable.
The Brimar longplate will have similar sonics to your Mullard; warm, laid-back, with a liquid midrange.
The Brimar longplate will have similar sonics to your Mullard; warm, laid-back, with a liquid midrange.