In case you haven’t seen there are a few threads here:
Personally I like the actual tubes that come with the amp like yourself. I have some Mullards and some others that I ordered (will post the type when I can find it) that are NOS. To be honest I didn’t really think they were better than the stock tubes - maybe they need running in more to differentiate. Anyway I don’t want to waste the ones that I have so will wait until they stop performing well before I properly use the NOS tubes.
@drarifakhtar - JAN, 2019 rolled PC88s for the BHK250 w/ Herbies dampeners. If you search I posted pics; need to be careful on adding dampeners for the BHK 250/300; restrict them from touching door & interior wall. After ~400hrs they settled in lovely. I had ~1350hrs on the BHK250 and loved the sound with the PC88 tubes…
Pics: Has both PRE & AMP, Title is PRE
APR, 2019 - Just rolled PC88s for my new BHK300s w/ same Herbie dampeners. They have ~150hrs and are starting to unfold nicely.