Wanted to share some impressions on:
SQ Amperex (USA) 7308
CCa Siemens
6N23P (Specifically the vaunted 1975 Reflektor SWGP)

First of all, these are all excellent tubes and through various rolling exercises, my top 3 (I’ve used other tubes like RTC E188cc, Amperex Holland, Tele ECC88s, other varietals of Russian 6N1P-EVs).

Transparency:
CCa: I think that the CCa takes the prize here. Ruler flat, slight upper end extension, and a faithful reproduction of the source. Also, the most analytical of the lot. #1 Spot.
6N23P: My favorite though is the 6N23P. Mainly because I still think it’s very transparent, fairly flat, with a little bit of low extension beyond the CCa and a little less extended on the top than the CCa. Gives me the best of all worlds. Takes the #2 spot but only because I’m saying the CCa beats it out.
The 7308 was definitely the bass heavy tube among the 3. Good presentation on the lows and mids, and has the least high end extension. Overall an excellent tube. Takes the #3 spot for colorizing the sound the most.

Soundstage:
CCa: Excellent soundstage, but not super wide. Overall it does a great job of resolving the instruments to their respective position. The stage isn’t too deep though. Takes the #2 spot.
6N23P: The soundstage is just unreal.
Also, just incredible resolution. Wider and deeper stage than the CCa. Again my favorite and takes the #1 spot.
7308: Wasn’t the biggest fan of the 7308 soundstage in my setup. Definitely does a good job of resolving the various instruments and positions but a narrower stage than the top 2 above which is why it takes the #3 spot for me. Takes the #3 spot.

Dynamics:
CCa: Not the most dynamic tube of the 3. Super analytical and dry and in my opinion the most WYSIWYG tube of the lot. While it’s a dynamic tube, it just lacks a little bit of the depth of layers if you want the last word on dynamics. #3 spot.
6N23P: Super dynamic tube. Love just how much depth of tone, layers and just how quick and fast everything sounds regardless of what type of music I play. #1 spot.
7308: Definitely a close #2 to the 6N23P. Does a great job of layering various instruments, but I think takes a slight hit to be knocked down to #2 because of the least level of high end extension.

Anyways, all my opinion of course. YMMV.