When I used the 845 tii, they opened up a bit after about 30-40 hours. Not a huge difference, but bass improved, and a tiny bit of an edge on the upper mids seemed to smooth out.
I'd suggest getting used to the sound difference the 845 tii made and then swapping the 6SL7, assuming you have new production TS in there, now.
Really, I'd suggest targeting the 6SN7 in your SLP98. I find the bigger changes/improvements are generally had working from the front rather than from the back. You didn't say what you have in there, now, but, if they're stock new production EH, would be a good place to start, particularly on the front two. Unless you are one of the few folks that like 'em.
But, if you go with the 300B, next, please post your impression!