Hi Lozz,
With the stock ehx ecc81 when I clamped down hard with the compression to get a fat over impressed tone to play with a light touch, I would find there to be more break up, which wasn't really what I wanted
With the mullard I've found it to remain much more 'firm' and 'fat' at these extreme settings.
Overall, including at very light compression, I would say the mullard is somewhat LESS grainy and in fact rather silky smooth and warm
These are my experiences and opinions of course but I hope that can help on your tone quest!
Bare in mind that using the pedal for warmth; you will have to have more compression for more tube and vice versa! My approach is this for some warm compression (always on) and a DHA vt1 for the more noticeable tubiness. (the mark bass comp's tube sound is in no way in your face and really is rather subtle - which I like).
Ash