Feel like I should jump in here, since I was having problems recently and couldn't find a solution on these forums or any.
I bought a used Amplifi 75 last summer, with the floorboard for 175$ on eBay. Seemed like an ok gamble for what I had read suffered from spotty connection issues. Worst case scenario I could sell the floorboard and use the speaker in my Trinity amp build, I figured.
Anyway, the thing worked great for months, until maybe November or so. I was busy with work and just didn't have the time or motivation to play.
I powered it up a few weeks ago and simply could not get it to connect to my iPhone XS. I made the phone forget the connection, re-installed the Amplifi remote app, did multiple reboots of both the amp and phone to no avail. I figured one of the multiple updates to my phone had made the app or connection protocol obsolete, since Line6 hasnt updated the app or firmware in years.
So I chalked it up as a loss, again figuring to salvage the speaker and sell the rest for parts.
Then I had to redo the electrical in my playing/work room, and the amp sat unplugged for a while.
Out of the blue today I figured I would give the thing one more try, since I did love the sheer multitude of effects and sounds the amp provided in such a small package. To my surprise, the first attempt at connection was a success. And, it stayed connected for the better part of an hour, with both my son and I taking turns playing.
I don't know if the days spent without connection to the mains allowed something to reset inside the amp, or if it had a premonition of being sold for parts, but the damn thing decided to work and work well at that.
Just thought I should share this for anyone who is having connection issues: unplug the thing for a few days and threaten to take it apart, and it might just start working as it should!
Side note: this will be my last Line6 product. I've been with them since the Pod 2.0, but this last experience has made it clear to me that the long-term support of their products is non-existent. For a professional who swaps gear every 3 months, that's fine, but for extended ownership, it is clear that Line6 is not the way to go.