The only thing I remember reading is that the E5/E3 would dammage the PL-50's because they are armature IEMs and lower quality amps would harm them.
But speakers should be fine, and they shouldn't harm the E5
E3 or E5 do not damage balanced armature based IEM. I have use them both with many BA based IEM without any problem. I have only heard of one confirmed case of a person's q-JAYS (dual BA) being damaged after testing an E3, and I suspect strongly it is because his player has DC leakage problem (reason as I too use E3 with q-JAYS, but never have any problem). E3 doesn't have a protection circuit to deal with DC leakage - if there is any DC leakage on the headphone jack (which should not), E3 will amplify the voltage and may cause damage to the headphone. Supposedly any DAP should have its own DC filter (two capacitors and two resistors) on the headphone-out, but in rare case a faulty capacitor will actually let a small voltage leaks out - it is usually small enough not to cause any problem until you connect it to an amp without its own DC filter. As for E5, there is a DC filter circuit in place so the same problem will not occur.
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