If anything, I wouldn’t mind a bit more tonal density in the midrange through upper bass, but not at the expense of clarity. I am also curious whether I will hear any spatial differences with NOS tubes, like greater perceived dimensionality.
From this description I would recommend Amperex. But you’ve tried one of the best tubes out there, Amperex PQ 7308. It presents lush mids smooth highs, and a 3D soundstage. Since you like the new production Mullards, it seems as though you don’t want extended highs since these are rolled off on top. I suggest trying NOS Mullard long plates with either square or round getter. If not available, Mullard box plates CV4003. Brimar is another brand from the UK, same time period, that have more clarity, are less warm with a good soundstage. I've seen box plates CV4003 for sale.
NOS Mullards don’t have extended highs like Amperex or Bugle Boys, but have more realistic highs than new production. They’re also more detailed, have a lush midrange, and 3D soundstage which can be holographic in the right preamp. The problem is price and availability. After the shortage of Russian tubes this past year, inventories of US and European tubes have been exhausted, so prices are very high. What I’ve been doing is buying slightly used tubes that test as NOS. Mullards, Amperex, Telefunken will still fetch a premium, but if they are ANOS (almost or about NOS) they could last years in a preamp. Brent Jessee would be the first dealer I would contact. Andy at Vintage Tube would have the best stock, but there’s a 6 week wait at this time. Use Google to check out prices. Maybe others can suggest some trusted dealers.