I tried 2 Philips/Mullard Blackplate M8100s (NOS) in the DARKVOICE 332 today (the M8100 is a high quality military version of the EF95, which is an equivalent to the Chinese 6J1). I bought 5 of these little beauties on E-Bay for £2.95, yes you read right, £2.95 + £2.50 delivery. Elsewhere, they're selling for around £10 a peice.
These tubes are fantastic.
Though good before, everything about the sound is improved: bass, while not particularly deeper, seems much more solid and well defined; the midrange is even more open and detailed; while the treble has lost that glassy harshness, which was my only real criticism of the 332. The sound has a liquid nature now, hanging together better.
I've ordered some 6AK5Ws, again from E-Bay (another mil-spec version of the Communist Block 6J1). I've also ordered some Svetlana 6S19P-Bs to try. I think the makers of the DARKVOICE could have picked a more widely available tube. So far, I can only find the Soviet versions of this valve - the 6S19s range and its military cousins with the "P-B" suffix: there appears to be no western equivalent.