I am in the same situiation. I've an iPad 2 and I'm not able to update my iOS further than 9.3.5.
I've already tried to search for AMPLIFi Remote 2.50 because is compatible with iOS 9, but I had no lucky.
Any one, or even you Line 6, have a solution? I want so bad to play my guitar and use my AMPLIFi 75...