The first step in fitting a hearing aid is selecting the fitting formula. This is the formula that determines how much gain is applied in the ear canal to address your loss. There are quite different approaches used in how loss is addressed. The most common industry standard is NAL-NL2. If one has a straight across 60 dB loss the correction would look like this. There are three curves because a different gain is applied to soft sounds compared to loud sounds. It is common to use compression which means soft sounds are amplified more than loud sounds. In this example the top curve is for soft sounds and the bottom for loud. The larger the difference the more compression.
Next is another industry standard called DSL v5. This is what it looks like for the same straight across 60 dB loss. Notice that there is very little compression of soft sounds, and less compression of loud sounds. If this forumula is used the hearing aid is likely to sound quite different, even though it is exactly the same aid.
And last here is a proprietary Rexton fitting formula called SmartFit. It is different again, but closer to the NAL-NL2 which it is likely based on, but modified by Rexton.
The point is that these will all sound different, and if each hearing aid you compare uses a different formula then of course they will sound different, but it is not due to the hearing aid, but due to the formula.
And the last point is that if REM is not done then you have no idea what the curve really looks like. The computer generated gains are likely to be wildly off what they are supposed to be.
Seems to me that if you want to fairly compare different hearing aid brands, you should choose one standard formula and ensure that one is programmed and REM fitted to each aid. Then you are comparing apples to apples.