Edit: The seller described the tubes as "new nos" and illustrated them as such, so I'm a bit surprised at the heat discolored legs. What must be considered when buying to get really unused old goods? Or is only the delivery of used tubes to be expected and I think too much?
Yes, that is a bit of a raw look, but I've seen it before on new tubes. It is part of the manufacturing process. There are several steps when heat is applied to the tube envelope and depending on the sophistication of the process - machinary or hand done - the pins can show the effects of the heating.
Also, I noticed you are from Germany, and thought I would mention there are awesome locally made RFT tubes - made in Germany - you might have easy access to by local dealers. Also, some of the top tubes for audio were made in Germany: Telefunken. Siemans, RFT, Amperex, AEG, Lorenz, Philips and Philips Mini Watt, Cerberus, Tungsram, Valvo, RCA, Sylvania, and more.
Due to my lack of experience with tubes, I am not able to compare the sound of the xDuoo AT-20 with other tube amps, therefore, I limit myself to the following:
With the "RTF ecc82 12au7 foil getter" the freshness in the sound is retained, added is a welcome warmth and physicality, which positively complements the RME.
The "Gold Lion 12AU7 / ECC82 Genalex" sound, after 30 hours of break-in, comparatively smooth and less exciting.
Changing tubes is, also because of the submerged location on AT-20, a bit of a hassle, making comparisons unpleasant.
That was a very good explanation, "less exciting" says it all.
You will gain your words to explain details as you read others reviews and comments, but really it is always best in ones own simplest terms. In the end it comes down to a few simple words to explain the sound. And, whether you enjoy it or not.
I use a microfiber cloth on cold tubes to insert and pull them out. For hot tubes - turn off the amp before pulling them out or inserting new tubes! - I use a medium thick kitchen dishes glove, not a used one, and it is much easier to grab onto the tube without much pressure and to rock it a little to the sides to loosen it and it pops right out. It doesn't matter what size the glove is, I don't put it on, I simply wrap the wrist of the glove over the top of the tube and use my fingers to hold up the slack and direct the tube into the tube socket with the correct orientation and then click into the key slot and push down. I use the Glove like the Microfiber cloth to get leverage and a better grip on the cold or hot tube than my hand provides alone.
Right now I have had the Mullard Holland E180CC tubes in mostly, as I need to return a couple of the newer tubes I've gotten due to IEM HISS and Static Noise, 2 sets on my TA-20 and one Tube on the TA-26.
One set I can only return, another I am sending back to swap for another pair - he's going to screen them for noise, and another tube for my TA-26 also had the static crashes. and he is also going to screen for a noise free tube.
I hadn't had a problem with noisy tubes for quite a while - since I only started buying tubes with measurements 100+/100%+, and then this last batch I ran into 3 sets with HISS and random noise problems. Fortunately I was able to return them all. It is important to only buy from sellers that allow returns, and even better those that will work with you to help you find what you need.
I should also mention that I don't usually hear such noise in my headphones, even with old used tubes, it is when I switch to IEMs that I will hear HISS on used tubes that have been "used up" to the point they start emitting high frequency hash noise, or HISS.
This time the static crashes of noise I was hearing were coming through on my headphones too, it was very unusual. But, for all 3 pair/single the noise was only heard with those tubes, not with any other of my NIB (new in box) tubes, or used tubes measuring greater than 100%+/100%+.
I'm happy you are enjoying your Xduoo TA-20, Rock on!