[pruss]

Well, with regard to safety, I now have 60000+ downloads in Google Play. A pretty good test bed, albeit mostly for phones and not the Fire. I assume that serious damage to a phone would lead to a one- or two-star reviews. I just skimmed through the one- and two-star reviews (outnumbered 5:1 by the five-star ones) and didn't notice any that allege hardware damage. One says it caused "issues" and one that it caused staticky noise (I think only when in use; I am glad the user discontinued use), and there were a few crashes, but most just complained of it not boosting volume (some I suspect because they were testing with in-call volume rather than media, and others due to some incompatibility).

One four-star review says that the user blew out a set of Samsung earphones by boosting with 100% and turning off the non-uniform-boost safety feature, but the user still seems happy after buying a new set of earphones. :-) Some users report violating the 40% rule with impunity.

We've been watching Amazon prime movies on my wife's Fire for the last couple of days, typically using a boost of 50%, with no problems, and excellent voice understandability due to the boost being focused on mid-range frequencies. This isn't with Speaker Boost but with an unreleased version of VolumeSwipe, but it uses the same boost method.

There are 1590 downloads in the Amazon Appstore, most of them I am guessing for the Fire. There are only four reviews, but they're all five star, with nobody alleging damage.