You are going to get replies on both sides. There is no way for you to know which you prefer unless you compare them. I would suggest that you borrow a SP17 or LS17 from a dealer. The other option would be to take your SP8 to a dealer and compare them.
I owned a SP8 many years ago along with a couple of other ARC preamps. I think that ARC's recent preamps, especially the one using the 6H30 tube are a huge improvement. When I traded my LS2 for a LS25 mk2 I was amazed at what a huge improvement there was.