The SP8 has an MM phono stage. It is not meant to be used for MCs. If you do not have enough gain on the input of the LS-17 from the SP-8's tape outputs, then get a phono stage that is appropriate for your cartridge. DO NOT use the preamp outputs or you have all of the SP-8 line stage circuitry as noted above. The SP-8 along with the SP-10 had horrible line stage performance.
The SP-8 is a great product but has limited usage. You can easily sell it and get a PH1, PH2, PH3 or PH3SE for the SP-8 price or even less! And your problem of gain will be solved and ultimately far better results.
There are other ARC line stages, most notably the LS5 and LS25 Mk I, that sell for much less than the LS17 and would significantly destroy the LS17 in musical performance. Give these consideration as well if you want to stay in the ARC domain.
The SP-8 is a great product but has limited usage. You can easily sell it and get a PH1, PH2, PH3 or PH3SE for the SP-8 price or even less! And your problem of gain will be solved and ultimately far better results.
There are other ARC line stages, most notably the LS5 and LS25 Mk I, that sell for much less than the LS17 and would significantly destroy the LS17 in musical performance. Give these consideration as well if you want to stay in the ARC domain.