Probably not. I have tried using my 150 at practice, just me and a drummer. It wasn’t loud enough for us. It’s the quietest 150 watt amp I’ve ever heard. YMMV but the 150!is too quiet for me, I doubt the 75 would work live. BUT, depending on the type of music you play, whether you mic the amp, etc, it might work for you.
No good reason and this is a violation of one of Apple's iOS App Store guidelines (rule 5.1.1 (ii) states " If your app doesn’t include significant account-based features, let people use it without a log-in. Apps may not require users to enter personal information to function, except when directly relevant to the core functionality of the app or required by law.") so I would report them to App Store Support. Pretty disappointing after paying several hundred dollars for the amp.