I bought this combo bluetooth receiver/amplifier on Amazon:


I tested it out with a quality pair of speakers, and it sounded great and had plently of volume. Once I installed it in my LR3, I encountered some nasty distortion whenever the music got loud. I was able to get a little bit of extra volume before distorting, but it was still definitely not close to CD level.

I suspect two things are going on with the aux input at the head unit. There is a pad that is basically lowering the volume level, and there is some sort of limiter that prevents the volume from going above a set level.

All that to say, I'm not conviced it's possible to get a whole lot of extra volume out of the aux input. Somebody please prove me wrong!

 

I have no distortion using the aux input. And I play music pretty dang loud. I use a direct wire from iPhone X (so I need a headphone adapter). But I hate the wire, its always getting caught on stuff.

 

DakotaTraveler, with your music playing dang loud via the aux input, what would happen if you switched to radio? Would the music now be DANG loud?

I think the issue that most people are experiencing is that using the aux input requires turning up the volume on their phone, and also turning up the volume on the Harmon Kardon head unit beyond what is reasonable for listening to FM radio or CDs. The obvious solution would be a line level amplifier to help boost the signal to the aux input, but my suspicion is that there is both a pad and a limiter being used at the head unit to prevent the volume from getting too loud.

 

Yes, they inputs are much louder at the same volume setting. But you said distortions, I am not getting any. The volume difference is well known and people have used mini-amps to boost input with no issue too.