Put RCAs in your American amps and Mullards in your British amps. If you only have ONE spot to put in an RCA blackplate , it would be a hard choice between V1 and V3/PI to the output tubes. I personally run all blackplates in my American amp and RCA smoked grey glass outputs. No going back, the sound is unmatched in tube terms.
Again, the 3D soundscape and smooth top of the Mullards combined with a strong, tight bass, makes it a dream in Marshall and Vox, etc.
I know a very reliable tube guy who happens to also sell on Reverb. I think it is Cool Factor audio. I always deal with him personally and Greg has top notch, quiet NOS tubes.