Last year I bought a TaoTronics TT-BR06 Bluetooth Receiver to use with my iPhone. It works great, and the sound quality is fine. The only issue was that the volume was too loud. I am using it with a pair of Sony XBA-N3 earphone, it was uncomfortably loud even on the lowest volume.
Recently I looked up for this product on Amazon, and I found a review that mentions a modification that can adjust its output gain. I tried it and it worked like a charm. So I’m documenting this mod here. Maybe it also applies to other similar brands of Bluetooth receivers. So if you have similar problems, you can have a try.