There are other tubes that can be used. There was a discussion about this on the audioasylum. Check the digital drive chat room for the date of September 5, 2002.
No, the 6922/6DJ8 tubes cannot be swapped with the 6H30 or vice versa. There are significant differences in most cases. Several tube parameters are sufficiently different in that the tube would not perform well. Either swap would require circuit changes and, in the case of the 6H30, given its very high Gm (for one) would likely have parasitic oscillations.
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