When the solo is written in a lower manual part, it will be played with the left hand on its own manual. This is usually a tenor-range melody.
If your solo stop or combination is to sound in that same tenor range, build on an 8' foundation.
If you want your tenor solo to sound in the soprano range, build on a 4' foundation.
If you build on a 16' foundation, it will change the sounding range of a tenor solo to bass, which is rather uncommon.