I bought a NOS mid-sixties, British-made 'Mullard' ECC83 (12AX7) for my 'Vox Cambridge RT 30' and it´s fabulous. Yes, it was very expensive, but I wanted my 'Vox' to have the same tubes that 'Jenning' used in the sixties. To me, 'Mullards' are the best tubes one can find.
Well, I believe I paid 100 USD for it, maybe a little more. If you want me to, I can check where I actually bought it. Can´t remember the name of the shop right now, but the owner was very knowledgeable and helpful.
Oh yes, I do believe they are worth the extra money, Bob. I read about the NOS ECC83 'Mullards' before buying mine and everything is true. To me, the 'Mullard' ECC83 it´s the best tube ever made of the 12AX7 type. Don´t know about the 12DW7, though.