HPA In Ear : Needs a very strong line-level signal to be loud enough

You have to feed the HPA a very, very strong line-level signal. Only then will full volume be almost too loud for your ears in your buds, and you can therefore turn it down a little and have some control over the loudness level.

But if you feed it an average line-level signal, you'll have to keep the volume knob at full, and still barely hear you're playing above the stage noise.

Does 9V power mean you can put this on your guitar pedalboard? Not necessarily - it's bloody huge. If this is your intention, measure it very carefully, and allow for plenty of access in-front (knobs) and behind (cables).

Build quality is good. Sound quality is good, and no hiss or hum using my pedalboard power.

Apart from the amplification issue, this is a nice wired-IEM unit.