Yours should be mullard rebranded with Philips.
Those are my military ones for reference.
They will get a listen tonight
RCA 6as7g warm, punchy , cheap and available. Less resolution than the others.
Tungsol 6as7g is very warm and holographic sounding. But they got quite rare these days. Expert tip.
GEC 6080 is more easily obtainable, slightly warm with good staging, seperation and resolution. Price performance is best.
GEC 6as7g would be holy grail of warm tubes with strong rarity and price. Still slightly behind 5998 in technicalities I must say.
5998 (or 421a) get these and be happy with the best power tube available in your slot. It's the least warm tube of those I mentioned.