I have a Saga+ which, as you likely know, has a passive mode which only acts as a volume control, and an active mode which uses the tube as a unity gain buffer. The tube that came with my preamp is incredibly neutral. You have to listen very closely to spot a difference between the passive and active modes, though the active mode is very slightly smoother, though with no loss of highs or dynamics.
Personally, I'm not going to spend any time tube rolling. I like the sound I'm getting from the Saga as-is.