Finally got my latest ebay tube purchase yesterday. 10 used EF91 type tubes for 9 euros, not too shabby I think; it'll keep me entertained for longer than 2 beers at the pub or a movie theater ticket for the same price!
Anyway time for my EF91 shootout. Out of the 10 tubes, 6 turned out to be "pairable", 4 being virtually identical. There were no actual EF91 tubes in the pile, but only CV138 and CV4014 versions, so all UK military (which is why I bid on this in the first place).
First pair I tried were some blue glass CV138 KB/D (crowbar, 6AM6, no logo). Deciphering the old military markings, these should have been made by mullard in 1953 in the Mitcham factory. Both tubes were identical and a had a slightly elevated noisefloor that didn't really impair listening. They're like these ones (the ones of the picture with "6AM6" also written).
These tubes were actually quite surprising in clarity and transparency, with shimmering natural highs and very a large soundstage with great separation (more wide than deep though, unlike the CV4015/M8161 which I find more holographic and "3D-like"). Bass is a bit loose like all EF91 tubes, but goes quite deep and is great for non-fast music (absolutely great for jazz, less with electronic music). Mids are fine but uninspiring unlike the best EF95 tubes. Overall, one of the most neutral tubes I've tried, but not quite as musical and detailed as the CV4015; the treble-centric presentation can get tiring very quickly though, and the CV138 can sound quite SS-like, which many might not like. I would only use these occasionally for specific genres of music.
Second, I tried two blue glass CV4014 KB/D (crowbar, 6064, no logo) tubes (special quality EF91, more ruggedized on paper than CV138, which it more or less replaces). Military codes supposedly show they were made by Mullard in 1956 in Mitcham factory (which is weird since the literature says these special quality types CV4xxx were made from the late 50s onwards). They are almost identical to the CV138 above in construction but an additional "cage" element below the getter than effectively hides the top of the filament that was visible on the CV138 -so no glow really. They look somewhat like these.
Logically these tubes should be better than the CV138 from the same decade, and they are quite nice and very similar, shimmering detailed treble, loose jazzy bass... But somehow, I just couldn't enjoy these the same way I did the CV138. There is an ever-so-slight difference that is difficult to pinpoint that makes the sound of these tubes less inspiring and musical (funny thing is they seem to be cheaper to buy too), maybe colder or more neutral than the CV138. Overall, very good jazz and acoustic tube. I suspect that there are better CV4014 tubes out there, maybe from later decades with a different construction (if anyone is looking to sell a pair of 70s CV4014/M8083 with blue glass, do contact me).
Last tubes I paired and tried were some grey CV138 KB/EN (crowbar, no logo). Decrypting the codes, these are basically old-world ruggedized EF91 tubes made in 1951 by Thorn-AEI in the Sunderland factory for the UK military.
One of the tubes was clearly weaker than the other one, so I'll never really use these for serious listening, but these have a very interesting sound, unlike other EF91 tubes I've tried. Bass is full and pleasant, even though it doesn't go down very low. Highs are still very detailed and natural, but not sparkly or sibilant like many EF91 tubes. Soundstage is quite large and instruments are well separated. Overall, these tubes are like a laid-back version of the mullard EF91 tubes, just less treble and fatter bass (but not bloated), possibly less detailed and not quite as good at staging as the two pairs reviewed above. Very pleasant for calm jazz listening. I'd love to find a cheap pair of these tubes NOS!
Anyway, if you're looking for absolutely neutral or "cold" tubes, the military EF91 variants seem to be a very good choice if you can live with more treble and looser bass.