I've no experience with
those particular ones in particular myself. The Brimar CV1988s that some of us use here are
these.
Those are an improvement over stock and great all rounders but personally I prefer the RCA Red Base 5692s which are
these.
A
used pair also.
We are reliably informed on this thread by more experienced people that Hytron versions of the RCA tube are exactly the same tubes just branded differently. You might be able to find some of those on eBay as well and save a few hundred £.
I really like the RCA Red Base tubes but they are just considered "good" and "nothing special" as well so really YMMV
Melz 1578s are great and my other favourite but
silly expensive and seem to vary according to year of manufacture. I was very lucky with mine. A bit of a luck is sometimes required and the prices for some NOS is simply ridiculous. I am curious about
these personally.
As others have said - scanning the
6SN7 thread is a good option and maybe
this.
I also knew nothing when I bought my Envy - I think you have to try a few out. The PS Vanes really aren't that bad either.