The 6DJ8/6922 has become one of the most popular audio tubes on the planet. With it's high gain and high Mu it's no wonder. There are many substitutes and equivalents for this tube and it can get quite confusing. All of the tubes we mention here are interchangeable and are compatible with each other.
The 6DJ8 is also called the ECC88 in Europe. This is the tube that all of the other equivalent tubes are based on. This is essentially the base model, time-tested, reliable, inexpensive and plentiful.
The 6922 is the up-rated version of the 6DJ8 which was made for audio tubes. These are also called E88CC in Europe. These tubes usually have gold pins and a longer life rating than the 6DJ8.
This is the most sought after and highest tier tube in terms of quality. Also known as the CV4108 in Europe. The 7308 is basically just a balanced and higher testing version of the 6922. The only difference between them is that the 7308 is truly cream of the crop.
This is the Russian version of the 6922 and they have gained quite the cult following. In fact, the 1975 Reflecktor silver shield version is said to be even better than the 7308. You may see these listed with Russian characters but the correct English designation is 6N23P. You may see them as 6H23N elsewhere.
Not so close equivalents
These tubes are usually interchangeable with the 6DJ8 family of tubes but they aren't close enough to call direct substitutes so your mileage may vary: PCC88, 7DJ8, 6BQ7, 6BZ7, 6AQ8, 6CG7, 6FQ7, E188CC.