Shure Beta 58A for vocals
SM 137 for instruments (dobro has his own mic, not sure what it is)
The Shure SM137 looks like it could be okay for this application, but you'd need to have it close to the guitar and be careful that it can't see the L1®. That is, the line of sight from the microphone to the L1® should be obscured by the guitar player's body.
What kind of feedback are you hearing; a high-pitched squeal or a lower-pitched howl?
From your sketch, it looks like the SM137 is the fourth microphone connected to the T1®. The T1® has only 3 XLR inputs. How are you connecting the SM137 to the T1®?
PS. I was told a few times that the L1 wouldn't work with so many microphones. That the L1 was made for "plugging in"?
Well... this depends on many things like:
- the volume of the voices and instruments
- the number of microphones
- the type of microphones
- microphone technique and positioning relative to the sound source
- the volume required for the audience
- the position of the L1® in relation to the microphones
Here are some tips.