Tube types

The E188CC was my favorite. I think I still have a couple, in my stash, as well as a couple Amperex. After listening to a few different CCa’s, the grey shields became my nine-pin miniature of choice. My CDP takes six. Glad I bought a few spares, before the supply dwindled and the prices increased.

I also(as wlutke) owned an ARC SP-9, some decades back.    I ended up with a Siemens E188CC, in it's main gain circuit.   Hadn't tried a CCa yet, back then.    Every tube, tried in that pre(there were a number), made a distinct difference in the system's presentation.  Of course, some more profound, and better/worse, than others    Again: so many variables!

@jea48- I mostly agree with Joe S, as well. I’m guess he didn’t have any late 50’s-early 60’s, grey shield, CCas. They command a premium price, for a reason. There’s much more difference, in the presentation of the upper-crust Siemens tubes, between their years of manufacture, than he mentions. They vary at least as widely as the Amperex iterations, which he seemed to have in abundance. So many variables, in this aspect of Audio!

"Have you guys found that switching to premium tubes has been a significant improvement?"       Absolutely!     Less painful, when you only need one.

You may find these interesting: https://www.head-fi.org/threads/6922-tube-review-17-top-6922-6n23p-e88cc-cca-7308-e188cc-tubes.761078/ and: https://wallofsound.ca/audioreviews/vacuum-tube-shootout-6dj8-types-part-1/ Keep in mind; there are a plethora of variables(individual tastes/biases/aural acuity, speaker system response/accuracy, electronics, etc) ie: Read Paulo’s post(Wall of Sound site), with which I’d have to agree, 100%(far as audio equipment comments).