The reason I say the amplified headphones might not work is because I tried to plug mine into the mic plug on my computer. I was trying to record watergates and get my TX audio in there. Just didn't work and I really don't know why.
Loudness will be from pep power more so than deadkey. The one i use works great behind my ranger 296 at a 2 watt deadkey and 8 pep. I actually have to turn it down a bit. I have put it behind a 100 watt radio and it works fine. Just turn it down first or your ears get a nasty surprise.
Unless you typed something wrong the reason it didn't work was because you plugged HEADPHONES into a MIC INPUT of your computer.