tough call between going to ref10 phono stage vs the ref 6 linestage
hard to know which will make more of a difference, though just a guess, knowing the arc stuff pretty well, the ls26 to ref 6 move may yield more of a step change (more transparency, snap and speed) ... i think it really depends how well your ph8 is mating with your current cartridge
good luck, your system is already quite excellent, as others have already noted