300b vs. NOS mullard el34- both amps in push pull

what kind of el34's are you using in your quickie now?
if you are into the distinctive el34 midrange sound, there are a few (cheaper) options to get very close to THE sound other than shelling out some chum for nos mullards; as Khaki8 has pointed out, there are some good KT77's and a few other current production 34's.

if you want to take advantage of your quickie's tube-rolling capacity and venture out to different tube sounds, there are some 7581A's, KT-88's and 90's, various 6L6's or even el84's using yellow jacket converters.

to my ears, many el 34's lack "air" and deep/wide bass in comparison to, say a nice set of tall-bottled RCA 6L6GC's or sparkly top of JAN 7581A's.
you might want to try out EL84's w/ yellow jackets; they have very good midrange (similar to 34's but a bit airier in the mids and have more brilliant treble, in general). plus el84's are cheap. i like groove tube's (selected JJ's)high number (6 and up) sets. these have more clean headroom than the lower numbered ones.

if it was my money i'd rahter save for a good 300b set amp than pp.
or even try some single-ended triode (MODE) versions of EL84's for a lot less $$.

yeah, el84's do lack power and headroom of the big bottles.
i can see why you want to try NOS Mullards, though many of the remaining nos tubes can be/are bottom of the barrel stuff and the performance can be disappointing.

it is hard to describe subtle tonal differences between several variants of 6L6's but i'd say 6L6GC variants in general have broader and airier sound than EL34's, and are different animals in terms of midrange presentation/tonal characteristics from El34's.

good luck w/ your search.