Some impressions on what I rolled through the Aune T1 so far:
SIEMENS E88CC - German Military (BWB) Selected Stock
Gold pins, Halo Getter
I read a lot about these Siemens tubes. A lot of people seem to be very fond of it. To my surprise it turned out to be my least favorite one so far. It does not sound much like an improvement over the stock 6922EH that came with the Aune. Since this came from a sealed NOS stock - maybe it needs burning in? Maybe I got the wrong version? Maybe my ears are just not good enough? Right now it sounds a bit muddy and dull, while a bit warmer than the 6922EH. I will get back to this again in a few weeks and give this some burn in.
Telefunken E88CC
Gold pins, Halo getter
Unlike with the Siemens above, I got exactly what I expected from the hype around this tube. Clear soundstage and a very open and rich sound. With my HE-400 it may already be a bit too bright for my taste.
PHILIPS MINIWATT E88CC SQ SPECIAL QUALITY
Gold pins, Halo getter. While the label claims "Made in Holland" it was actually made by Mullard in their Mitcham military factory in the UK.
By far my favorite tube to go with the Aune. It is almost as clear as the Telefunken but just a slight bit warmer, smoother. Vocals sound more pronounced. Highs and lows are slightly rolled off and not as sharp as on the Telefunken. Maybe too much for some - but perfect for me and my HE-400. I love this tube and it is what I use for listening most of the time now.
I found this one locally for a laughable price of €5. Looking to get some backup stock on eBay it turns out that it is apparently one of the most sought after Mullard tubes made. eBay prices seem to reach up to well over $150-200. Well damn. I am almost too afraid to "use it up" now. I hope I can find some more.
AEG E88CC - German Military (BWB) Selected Stock
Gold pins, Dimpled plate getter. AEG label, made by Mullard in the UK.
Another tube I got from the same German military stock where I got the Siemens from. As it turns out it was also made by Mullard in the same factory as my Philips Miniwatt above. As a result it sounds almost exactly the same with slightly more clouded instrument separation and overall a bit less smooth. This probably needs some burn in. I will get back to it later.
In summary - I seem to be very fond of the British Mullard sound. I will try to get my hands on some more Mullard tubes to see how they compare, but so far they are my clear favorites.