Bandshells help sounds that are omnidirectional. The bandshell focuses the sound that would normally go backward and disperse - and sends it forward to the audience. The result is a supportive early reflection that reinforces the sound source.
Your L1 Compact and S1 Pro are already directing sound forward (and very little backward), so the bandshell probably won't help much.
My trio (3 vocals to T1 , 2 guitars to amps, & 1 bass to amp) have been using an L1 Compact and an S1 Pro mainly for vocals, with instruments to separate amps. Typical scenario is up to 200 people in quiet indoor space.
Thanks for the background information.
We will be playing soon in a band shell with upswept seating for maybe 600.
I would consider doing this with an L1 Model II, but not with an L1 Compact plus S1 Pro.
Will sound projection from the band shell make our L1 Compact/S1 adequate, or should we use a more powerful system?(we have a more powerful conventional P.A. available)
Take the more powerful PA.