Spiro weichs work just fine. You could go the downtuned solo route if you need to, but I don't find the weichs too heavy for slapping, on either of my basses.
At the end of the day, it's up to your bass what sort of tension you need
A Spiro weich E/A plain gut D/G thing works well as "do-all" setup too, if you need things to be a little more old-school sounding. Just be patient and let the Spiros break in...and commit to some time in the 'shed to learn how to bow the guts.