ARC SP-8 to SP-17?

The appearance of a couple of SP-17s (and at least one LS-17) for sale over the past month made me day-dream a bit (until I get my taxes paid, this is probably all that I can do for now!). I'm using an SP-8 pre amp, re-tubed and re-capped by ARC in 2009, driving an ARC 150.2. What I wonder is whether swapping the older pre amp for the newer one would be a real improvement, or a very minor improvement, or a lateral. While the obvious test would be to try it, that isn't easy to arrange. The route which seems to be a more likely improvement would be to go for an LS17 plus something like a PH5 phono stage, but that is quite a bit more costly. Any thoughts from you ARC users out there?