tube recommendation please

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.

There is no best tube, only a best tube that will allow you to achieve the sound that you want. I would suggest living with the pre-amp for a while and then describe in detail what attributes you would like to improve. If you find that your current tube set up is too lush and warm you will get remarkably different tube suggestion if you had posted too dry and analytical.

Since you don't have a tester your better off buying matched pairs from one source not trying to match single tubes that have values from different testers.