As Spendorman says generally.
The later ones had some changes such as different output transformer spec which were designed to cut the stress on the EL84's and prevent any risk of thermal runaway in them...
Earlier type probably better sound quality as higher HT used and greater amount of screen grid feedback. early ones likely to be harder on the output valves and will need decent ones fitted for them to work for longer than a few minutes in the worst cases...
I recommend Russian 6p14EB (exact name will very depending on translation of the cyrillic) as a good sturdy replacement for EL84s.
Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)
Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
Digital: Yamaha CDR-HD1500 (Digital Swiss army knife-CD recorder, player, hard drive, DAC and ADC in one), PC files via 24/96 sound card and SPDIF, modded Philips CD850, modded Philips CD104, modded DPA Little Bit DAC. Sennheiser HD580 cans with Arkless Headphone amp.
Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco