The military grade (DR NOS) 6H30 is a truly great tube. These were manufactured pre Y2K (year 2000) and are scarce and very expensive ($200 - $300 each). That said, the quality of current production is mediocre (sound and durability). There is a large market for high-quality 6H30s and I've mentioned this to the new Western Electric tube factory in Georgia (U.S.A.) (no reply).
The basic design of the 6H30 is ideal for audio.
The basic design of the 6H30 is ideal for audio.