So I bought the Willsenton R-800i

I would listen to the amp with stock tubes for at least several months in order to learn its sonic signature (especially because you already love it), and then would start tube rolling - slowly, pair by pair. Just live with acertain pair of tubes for a while and listen to the music. Hectic tube-rolling would not bring you (and me) much pleasure...

Thanks for your replies!

@2psyop I have no thermometer at hand here, at least none I could use for measuring the amp´s heat. All I can say is yes, it does produce warmth or heat, but even after running for an hour you can touch the cage (above the tubes) without getting burned immediately.

@unison77 You are right there, but I´m impatient. I can always go back to the stock tubes once I hear how it sounds with others. Maybe some of the outlier tubes I bought will sound crappy and the stock ones turn out to be great in comparison, who knows.

Unfortunately without any reference point it will be hard for me to describe the sound of the amp accurately. I mean, that´s always the problem when people describe their impressions: Some of the terminology is so vague and perception to a good part individual, you always have to read what people say. Then factors like sound sources, the room, age of the listener, etc come on top.
What I can say is that compared to my old Rotel RA-03 the sound is tremendously different: The warmth is there, that I so desired, the soundstage opened. It´s like the music (especially voices) are not coming from the speakers anymore but from somewhere between them. The bass is more pronounced than before, in a pleasant way. As for the highs: I think it could improve there. On this soundstage they feel a bit like behind a curtain, just a bit. The audiophile bit that when I listen casually doesn´t matter.

One thing I have to point out that I found not expressed anywhere else: There is music, that will not work well with this amp and I guess tube amps in general. I tried Alva Noto´s latest record „Hybr:ID 1“: It´s super clean, almost surgical electronic music from Germany (he made great records with Ryuichi Sakamoto), there are high buzzes and beeping sounds paired with nice rolling bass. It feels like my stereo tries to put a human warmth in there but it struggles, it can´t. Which means it´s not designed to reproduce this kind of music the way the music asks for. And that will extend to other kinds of music too. Some brickwalled/LUFS-optimized pop and electronic music. Oh yes, I tried some current Drum´n´Bass – nah.

Here´s some music I know quite well that gained a new level of sonic expressiveness with the amp, ie: gave me much pleasure:
The Cranberries „No need to argue“ (the presence of the voice right at the beginning – I guess that will be my demo track for the stereo)
Stereolab „Margerine Eclipse“
40 Watt Sun „Perfect Light“
Bremer/McCoy „Natten“
Low „C´Mon“
Yob „Our raw heart“
Calexico „Feast of wire“
more to come...

chmaiwald. Very nice comments on the sound of the amp. I would be very surprised if you were not satisfied with that amp. Further down the road you can look at other tubes. I have replaced my output tubes with Genelex Gold Lion and the preamp tubes with NOS GE and Sylvania. That took my Line Magnetic up from good to very good. But I have heard Mullard, Brimar and Telefunken have a stellar sonic signature. It’s refreshing to know the amp does not get too hot. When I switched from SS to tubes I couldn’t go back. It’s either an all tube system or tubes on the preamp side and SS amp. I really like the old warm sound. To me it’s organic and pure...

The Chrome browser translates foreign language web pages. I use it a lot for Italian pages.

I've always wondered about after sale support from the likes Wlllsenton, Cayin, and multiple seemingly well regarded amps that don't have wide distribution in the USA.   That's the major reason I stayed away from them.