The EL84 / 6BQ5 (in America) is one of the most common power amp tubes. It is commonly used in guitar amps, especially classic British tube amps. Most tube manufacturers have a version of the EL84 because of its popularity.
Compatibility - these tubes will work with any like for like EL84 power tube. This means if you have a JJ EL84 and want to replace it with an EHX EL84, that's absolutely fine! What you can't do is replace an EL84 with another power amp tube, say a 6L6.
However we do advise replacing full sets at a time so they match, and keeping the brand the same. A tube from a batch up to 15 years ago, even if it's the same brand, will likely be significantly unmated. It's not dangerous to ignore that advice, but for tonal reasons we think it's best!