Best tubes for the Pro-Ject Tube Box SE II Phono

Are thier any tubes that will out perform the stock tubes that this phono preamp comes with?
Any help will be appreciated...
I have the Tube Box SE and i really liked NOS Mullards but i want a warmer richer sound. I tried the GT reissue mullards but they sounded flat.
I tried Sylvania black plate 5751, Tung sol 5751 and ge triple mica black plate silver clip 5751. This was on a tube box ii, not the se, and all sounded much better then the tubes it came with. In my system I preferred the Tung sol.
Buy a few used pairs, play with them for a while and sell the ones you like the least.
By the way, what are the stock tubes?