Hi
@maketus,
@tomb gives good advice, especially with the three qualities which differentiate tubes, so well done
@tomb, maybe one day I would suggest set up a dedicated thread to explain this further if one isn't already available.
Just my humble two cents but before I myself go into too much detail, what tube amp are you using as this would help narrow things down to see what has been tried already.
But in general, most 6 Series tubes share
similar characteristics but are
not similar so be careful what you pick but in this case, you should be fine but don't quote me on that as aside from the three differing tube qualities as already mentioned, some tubes also come in different types or designations in the same series, the letter at the end of the number is there for a reason.
From personal experience, aside from doing research on compatible tubes which is quite easy nowadays due to the internet as was already stated by
@tomb, when choosing tube amps in any design, prior to purchase or even demo said tube amp, regardless whether you roll tubes or not, or just simply after a replacement tube(s), find out how easy it is to find said tubes used & how much they cost to replace or
'upgrade' as when potential
'upgraditis' hits, it hits some hard...much like anything in this hobby...
Though saying that, as
@tomb says, there are many good tubes out there, whether unicorn or not, which are really great to awesome sounding for the price, I have found one recently in particular myself which I am ridiculously happy with & hope to get more though I need a tube amp which supports more tubes though I am also considering buying spares in case they ever run out but I digress...
A good rule of thumb when choosing tubes, even those of the same series, is find out what general concensus says a particular tube sound like, not just changing said tube just because...
Vaccum &/or valve resources will usually give this information quite readily too, as do demos of said tube series as not all tubes, especially in a particular series sound the same.
Once you have a general idea what said replacement &/or upgrade tube might sound like then decide from there whether you want to hear & experience said tube.
This definitely makes things less confusing & time cosuming to find what will work for you.
Other places which may help you as well, simply entering search terms,
@maketus, is the following in no particular order :
thetubestore.com
tubes-store.com
Hope this all makes sense.
Hope you have a great day !
One of the best data resources on the web is the Duncan Tube Sheet Locator: 
