Without knowing all the particulars about the new s.e.x. amp kit, if the power transformer has enough oomph, it is possible that ths could be a good platform on which to build an SE el84 amp. I don't offhand know what the operating point would be with the 4k transformer, but 4k should be fine for an el84. Given that the opts don't have UL taps though, you'd have to run in triode or pure pentode mode, and they may not have enough current handling capacity for pentode mode.
Regarding thee Mullard circuit above (I'm assuming it's the Mullard 3-3), that amp was designed in the era of very high output ceramic cartridges and thus has an awful lot of gain and hence noise.
In terms of PP, the diytube board is nice, as is the Triode electronics one, and you can also check out the tube lab pp el84 amp.
-- Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)