First, you mention solid state. While the SP-5 was a solid state pre-amp, none of the later SP series were; all were tubed. Looking at later SP-series pre-amps, the SP-9 had two tubes, the SP-14 had three, and all the others had more. So there really isn't a solid-state ARC step-up in the line.
I had an SP-8 for many years, and used the built-in phono stage with my Linn LP12 as well. I thought it was a good unit, but when I upgraded to an LS17 (line stage, no phono section) and a PH5 phono section, it was a significant improvement all around (i.e. the LS17 sounded better than the SP-8, and the PH5 sounded better than the SP-8 phono section). Also, all of the older SP pre-amps were pretty high gain, which made using them with a CD player problematic, since the CD player already has a high input voltage relative to other inputs.
I had an SP-8 for many years, and used the built-in phono stage with my Linn LP12 as well. I thought it was a good unit, but when I upgraded to an LS17 (line stage, no phono section) and a PH5 phono section, it was a significant improvement all around (i.e. the LS17 sounded better than the SP-8, and the PH5 sounded better than the SP-8 phono section). Also, all of the older SP pre-amps were pretty high gain, which made using them with a CD player problematic, since the CD player already has a high input voltage relative to other inputs.