I have commented over and over here about the LS5 II and III and also the SP10. I owned the SP10 for 8 years in the 80s. It was a wonderful preamp for LPs but its mediocre line stage was not worthy of CD playback. And no doubt the line stage affected the ultimate performance for LPs too.
I changed to the LS5/PH2 combination which had all of the magical bloom of the middle octaves but brought on much refinements at the frequency extremes. And the SP10's high noise level as well as much older parts I am sure had much to do with its not very impressive resolution. The LS5/PH2 had much more low level detail. I owned this combination for 7 years until I moved onto the BAT VK31SE/VKP10.
The LS5 MK II is wonderful but it has an exaggerated top end, too much sibilance, which can be an annoyance once you hear the refined MK III. I had both in my home for a few weeks until I sold off the MK II. Contrary to what I have read by others, I heard no differences between these too in the mids, harmonic richness, ambience, etc. The MK III's top end is simply more natural and not fatiguing. But the MK II had a little more bass definition or extension. Whether this was accurate I can not answer. But in either case, since you are not using phono, the SP10 would absolutely be the wrong move here. I know of people who use the SP10 strictly as a phono stage and then drive it into another line stage.
The MK II has 30db of gain which only might be needed for phono users. The MK III comes either as 12db or 18db which is set at ARC or can be changed through resistor kits.
Stay away from the LS2, LS15, LS22. These are not at all in the same league as the LS5. All ARC preamps are not alike!!!