tube recommendation please

If the manufacturer could confirm this, that would be great, but I found a picture of your amp on the internet and it looks like the 12AX7 tubes (the pair in the front) are most likely the signal tubes and the 12AU7 tubes (the pair in the rear) are the driver tubes. The signal tubes will have the dominant sound and will be the most sensitive to tube microphonics. So, in terms of best bang for the buck, one way to look at it would be to replace the signal tubes first with the highest quality you can find, followed by the driver tubes later.

As Mofimadness mentioned, the Mullards are indeed warm and are fine tubes. If you find them too warm, RCA and Tung Sol are nice also, albeit not as warm as the Mullard.

Here's some examples of the degree to which you could fine tune your system, in order of decreasing warmth:

1. Signal & Driver both Mullard
2. Signal = Mullard; Driver = RCA or Tung Sol
3. Signal = RCA or Tung Sol; Driver = Mullard
4. Signal & Driver both RCA or Tung Sol

In my system I am currently running Telefunken for the signal tube and RCA for the driver tubes, although I am about to go all Telefunken. But Telefunken are on the neutral end of the spectrum, however, which is what I prefer.

But if you experiment with the Mullards, RCA, Tung Sol and Amperex tubes I'm confident you will find a satisfactory combination.

IMO the Mullards are hard to beat. Telefunken are also quite good and I agree w/ Reynolds853 in regards to neutrality.
Brimar/Amperex bugleboy have been good for me also but it's been a while since I've used them and memory can't always be relied upon...
This may be anathema but Groove tubes has a spiral filament
12ax7 that is supposed to emulate the Mullard, but I have only used them in a guitar amp. (Old Marshall- that originally came w/ cv series Mullards- I took them out so I didn't wear them out) The Groove tubes are a great substitute in that application and I really don't think the character of the amp changed much if any- I was smiling the entire time I played it which was true with the Mullard preamp tubes as well :)

You might find some added benefit in a bit of power conditioning. Power conditioning does exactly what you want. Smoother upper frequencies. Some do seem to lean out the midrange a bit (really making it tighter, and tighter bass, but it can seem as if that made the lower freq. leaner. IF you have a warm lower end, and don't mind it being made tighter, then i would really recommend a powerline onditioner for your electronics.
If you love the lower end exactly as is, it will be harder to find a great Powerline conditioner that will work for you.
this is only a suggestion to help you find the sound you seek. And i do not endorse any particular conditioner product.
I do own several (furman REF20i, Monster 7000SS, PS Audio P600, PPP), and power cords and also use quartz crystals for power conditioning

Thanks, great explanation on the signal tube versus driver tube. I will double confirm that 12ax7 is the signal tube. I also read some comments saying the rectifier may not have any tonal impact. I just order a pair of CBS-Hytron 5814A (12au7) from eBay based on some comments on the internet. I think I am good now. I will wait for the tube and see how much difference does it make. If the result is positive, then I will spend more money and get some nicer 12ax7, like the Amperex long plate D getter =)

BTW, I got a question in buying tube. On eBay, there are many single tubes. But most of the time people would need a match pair. The test score provided seems to be different depends on the instrument used. So .. if I buy a single tube, would it be very difficult to find one that match? It seems those score reflects the life of the tube. If I got un-matched tubes, does it means one may die earlier than the other? Or it will affect the sound in a negative way?

Thanks in advance.