From about 2011 to 2016 I went on a amp buying binge. All of the amps that arrived with EHX KT88 or 6550 had lot's of problems and pretty much all of the output tubes failed over a very short period of time. I replaced all of them with JJ KT88s and have not experience a single failure since.
I also bought a small sample of most of the 12AX7 tubes that are currently in production, to have some fun tube rolling. I rolled the EHX 12AX7 in a couple of bass amps and it was consistently the worst sounding tube IMHO. Here are my notes from when I rolled it in my Trace Elliot V8 OD channel:
The Electro Harmonix 12AX7 had the leanest/brightest timbre and produced the most uneven fizzy/buzzy distortion. The EH also had the least linear transition from clean to distortion and the break over into distortion varies with frequency. Low notes require a stronger signal to distort than higher notes.
IMHO the EHX12AX7 tube might be useful for high gain guitar use, where it's always pushed into heavy distortion. I have seen a few threads where people like them in bass amps as well, and I wouldn't be surprised if a few are hiding in my Reeves 400, which sounds fabulous. But in general, I give it a low rating.
I do think many of the EHX effects sound pretty awesome, although I don't own any of them. In particular I have loved the sound of the Small Stone since I was about 5. I bought a Blackout Effectors Whetstone which is able to produce a very similar sound, although it's not quite as wet.
I also think some versions of the EHX POG and HOG can make some interesting sounds. I don't have any effects that are similar to the POG or HOG. They would be fun to play around with, but I can't imagine a situation where I would use either live as a bass player.