AFAIK fitting range’s upper part only means that aid cannot ‘amplify in such small amount’, but that’s of concern only if person is around that top line, no?
But then, they wouldn’t look at SP but S/M aids.
If low frequency is that good/normal, then maybe vent is enough?
How the loss looks like?
I think that information could help people who are more knowledgable than me to chime in with proper answer.
I perceived bigger receiver as a thing where you go there when others don’t work for you.
I use P because it sounded better than M, but I could go M probably, but I was on the border (bottom one for low frequencies). But I didn’t study in much detail how it works.