without knowing a little more detail I can't be sure but I suspect that impedance mismatch is not your problem
A simple 58 and a standard Mic input arrangement should interface fine.
This box is likely to have an adequate line output and if you are using a line input to the mixer I think it should work
I suspect your feedback problem is caused by the use of compression in the presets you are using
Turn the compression OFF and try again
Then dial the compression back in slowly untill you find a workable level ...
This is a live sound / PA issue and experience is required