Why are you replacing them? The original Mullards are really nice if you can get them. As for new tubes the SED "Winged C" tubes are good sounding - much better than Sveltlana branded whatevers.
Are your amperex "worn out"? If not, then you will be spending a bundle to replace them. NOS are definitely better than anything new. I have played with most of the russian and chinese tubes and really like the resolution and detail of the svetlana el34's for the money. I have a really inexpensive used quad if you are just looking to play?