I was very upset as my ampli 150 became into sh** after updating via Bluetooth.
I read a lot of posts, and after 20 or more attempts it went very good. so I have my ampli back :)
so, Hereby I want to share with you .
1) look up for a spare iPhone (different that one where the update falied)
2) pair it, Install your app, and then flash it using a 2.50.2 version
3) I took a three hour nap and my ampli was alive again :)
apologies in advance for my bad english, Just wanted to share with you, in case it helps. This was very frustrating.