So I bought the Willsenton R-800i

Look into some of the simple DIY mods to those Cornwalls.  They really make quite a difference in SQ.

I have read a bit about it in the past and was not sure if I´m up for the task. Is there any mod you can receommend?

A short update here:

I have tested a few tubes and maybe you´d like to hear.

805 – stock replaced for Cossor
Judging from how they look the stock ones seem to be rebranded PSVane Hifi 805s. After listening to the Cossors for a few weeks I now put the stock tubes back in and I´m surprised that they also sound very good. Maybe a bit too boomy on the low end and with less stage, but tbh I still have to break them in properly, so for now they stay in.

6SN7 – stock replaced for Sylvania GTBs, waiting for Fonon NOS
One of the Fonons was broken. So I´m using the Sylvanias atm. Ordered some RCA NOS and of course I´m tempted to buy and test others but I´m holding back for now. It´s just costly and the Sylvanias are doing a great job. There´s the 300Bs and, probably, the 805s to be taken care of so I´m not trying to get carried away here.

12AX7 – stock replaced with current Mullard model, waiting for West German ones from AEG and Telefunken, East German ones from RSD and Funkwerk Erfurt (both used) and also Sylvania JAN 5751 NOS (a military model)
Still waiting for the west german ones. The stock one was JJ and just like the Sylvania and to a lesser extend the RSD it did a nice job. But the Mullard stands out for me in overall performance: Detailed highs and lows, lovely stage and nicely pronounced mids. Would like to hear NOS Mullards without having to buy them.

5U4G – stock, waiting for RCA NOS black plate and Svetlana NOS „Coke Bottle“
Currently the Svetlana is in and the overall sound seems a bit more transparent. But really, just a bit. Maybe I´m entering the world of audio voodoo because I want to hear a difference. But they also look better. Having read that 5U4Gs shouldn´t have much of an effect on the sound (if at all) I feel no need to look any further here. What a relief!

A few things I learned in the last weeks:

So many people write about audio with a presumed authority but considering their experience, their ears and taste on one side and ones own system, room, taste, ears and financial means on the other there is only so much you can take from it. You have to go your own way. And it´s ok not to end up with a system filled with Telefunkens with diamond bases, other NOS-tubes from that one specific year and whatnot. (By saying this I´m also reminding myself of it.) It´s also the structure of forums like this with people exchanging views about what´s better or not that implies you always have to hunt for the best.

Also, having a system now with much better resolution I also get to hear some mistakes or imperfections that were not so prominent before. Take „Roads“ by Portishead for example, a devastatingly beautiful song. It starts with a great organ intro that immediately pulls you in (down, some would say). Unlike before I now hear that on CD there´s some overdrive on the right chanel there. A few times the sound distorts. It´s bugging me like hell! I checked on spotify – the same. So maybe it´s everywhere and no mastering error. Still, it´s the first time I feel the strong bite of the sudiophile bug that wants to divert my attention from the song (which is still amazing) to the recording. Luckily he doesnt bite that often and I can still focus more on the overall improvement of the playback and a deeper connection to the music I listen to.

Lastly, for now, a question:
How much were tubes back in the day? They were so common but still needed skill and precision in production. It´s hard to imagine how much 12AX7s or 300Bs or 6SN7s used to cost when they were widely available. Does anyone know more or less? And I keep hearing that prices went up a lot in the last years for NOS. How much?

In order of importance to sound quality I'd rate 805 first, rectifier, than 300B. The Cossor is produced by Psvane, wouldn't expect much change from stock Psvane, Mullard NOS rectifier is one nice tube, use them in my custom 300B's, much greater reliability and longevity compared to modern, 300B, any number of top tier would be nice.

Assuming that amp up to it, top tier tubes will be impressive upgrade from present tubes. Crazy, but I could easily spend $2500 up on those three tube sets alone, even more crazy, it would be worth it.

I don't know what tubes cost back in day, assume pennies on the dollar, even accounting for inflation. NOS always going up, increasingly limited supply, likely greater demand over time, add inflation, prices up to be expected.

Assuming that amp up to it, top tier tubes will be impressive upgrade from present tubes. Crazy, but I could easily spend $2500 up on those three tube sets alone, even more crazy, it would be worth it.

Agree 100%. If it is a good quality SET build, then the upper tier DHT tubes will exploit its full sonic potential.

Charles