Your observation about SED's vs Mullards IMHO falls right into the same observations that can be made about equipment in general. Stuff (including tubes) that have a 'full bottom end' more often than not obscures some of the information present in the high frequencies. SED's have a tendency to be a bit forward in the upper mids but are not bottom heavy so you can get better dimensionality when using them. The Mullards probably are a bit 'warmer' than the SED's therefor the differences in imaging that you have noticed. Of the three new(er) EL34 production type tubes I enjoy more often than not would be the JJ E34L or NOS Tesla E34L (their predecessor).
FWIW, a tube I often use in lieu of a SED EL34 is the SED 6l6gc. A very linear tube (if that suits your needs) emphasizing neither the bass nor upper mid range emphasis of the other tubes you are using.
The bad news - SED's have been discontinued as a new production tube.
FWIW, a tube I often use in lieu of a SED EL34 is the SED 6l6gc. A very linear tube (if that suits your needs) emphasizing neither the bass nor upper mid range emphasis of the other tubes you are using.
The bad news - SED's have been discontinued as a new production tube.