It's a pre-amp section that is sensitive to noisy tubes. Very common. Call Upscale audio and they will set you up with some good choices.
^^^ this ^^^
new production mullards are not real old stock mullards, nothing to do with them other than the name, which has been purchased by the russian tube maker
you need real noise screening on the tubes, need to buy from someone who can do this... search other threads for old stock tube dealers - kevin deal is one of them, 4-5 others out there who do this properly