I think the short answer to your question is that holding your hands behind your ears does two things. First it reflects more sound into your ears, or in other words increases the amplitude. Second it shields the ear from reflected sound or noise. In the absence of the noise from the back, your ear can turn up the gain more and hear better.
That brings up the question of whether or not your rear speakers in your sound system are helping or hurting. At least in theory they should be helping if the arrival time of the sound is well tuned. However these sources of sound may be competing with reflected sound from the back walls and sides of the room. Some is good because it makes the sound more open and life like, but too much can make it hard to hear. Some hearing aids have an anti echo mode that helps to reduce this effect.
Rather than just changing gains, you may want to explore using a different fitting formula or prescription. There are a number of them that should be built into the software used to set up your hearing aids. The problem however with just changing the formula is that when you change it, then you should have real ear measurements done to verify that in your ear the HA is following the set prescription. Have you had real ear measurements done? It may just be an issue of the HA not being set up properly…