We have both "Behringer Powerplay PM1" and "Fischer Amps Mini Bodypack" as end pieces in a four-channel wired in-ear system. Since they both ought to be simple passive variable attenuators we thought we could use them interchangeably. But, the PM1 farts out at much lower levels than the Fischer Amps. Really ugly distortion at levels that was normal listening levels through the Fischer Amps. Since we had two of each, we could guess that the difference was by design and not due to malfunction.
Of course, we could compensate by applying proper gain staging to the signal running into the PM1. But as the Fischer Amps cope with higher input levels, my experience says that they are more capable than the PM1 to deliver clean sound with opportunity to higher signal to noise ratio.
The fact that the PM1 is able to distort at normal levels from an headphone amp makes them hard to use without taking special care.
(Our four channel system was powered with either one "Millenium HP4" or four "Behringer Powerplay P1". Source signal was fed from the Behringer S16 stage box or the X32 desk. The model of earpieces was Shure SE215 throughout.)