I've had some time to play the MP-1 with the two Mullards and I like them in both spots - a nice blend of gain and thickness. They don't have the fizz of the Tung-Sol, and the sound isn't as sharp and piercing as the other tubes I tried... the nice middle ground I was looking for. The Tung-Sol was just too much for V2, but the other tubes I tried there were weak and overly bright in comparison.
I must say, the Mullards strike a very nice balance in my MP-1 - thick (but not too thick) and gainy, but very clear and articulate at the same time.
It seems I can control their response better with my fingers and pick. The Tung-Sol was just over-the-top all the time in V2 and the other tubes lacked drive and bottom end. With the Mullard in V2 (and also V1) I can get close to the gain of the Tung-Sol, but not enough to get fizzy and blurry, and I can get the bite and sharpness of the lower gain tubes by 'choking up' on my pick with my thumb. Very responsive... makes me get more into it when I'm playing, because the amp is playing back.
Thanks for sharing the advice.