Signal tubes vs. current source tubes?

Can anyone help me understand the difference in sonic impact of the two different tube types in my BAT VK-5 preamp. The 6922s are the signal tubes and are often swapped out for alternatives to achieve different sound. However, some folks swap out the two 5881s, which can be more expensive, and talk about sonic benefits as well.

Where's the real bang for the buck? What's the fundamental difference between these two tube swap-outs? Is swapping out both tubes complimentary?

John B
Focus on the ones with the most gain , in this case the 6922's. the 5881's are probably not in the audio signal path. Check with victor@BAT, he posts on AA a lot.