Not necessarily Mittels! If the OP's bass is a Kay, it will be too much for the top. If he has played nothing but Innovation sets, then that is quite a leap in tension to get used to.
Spiros come in a variety of different sets All The Different Spiros Sets
Yes, a set of either S42 Weichs or either of the solo sets tuned down will be fine to start. Here's how to choose which set: starting with the current strings: how do you like the overall tension as the bass is strung now? If you like the overall tension, then the S43 4/4 solo set is the place to start. If you like a tad more tension, then the 3886 3/4 solo set is where to go, and if you like to "dig in" a little more on the E string, then the S42 4/4 is the set to choose. If you're ready to step up the tension to the next level, the 3885 3/4 Weich set is where to go.
More on the top: there is a balance: all else being equal, some increase in string tension will drive the top more efficiently, meaning louder, but beyond a certain point, the top will "choke" and not vibrate as freely. Since nothing else is said except the bass is a ply bass, if the top it thick, then the slightly heavier sets will be needed to get as much as possible out of the bass.
Finally, have you had the bass set up by a luthier? A proper setup overall will do more to get the most out of a bass than will any set of strings.
Epilogue: a lot of folks do switch out the G string as being too twangy for something else, but stick with it for awhile before you consider changing.