Carlos,

it's largER than the one I had before, where I didn't have problems like
this.

What's much more likely is you had many peaks and nulls at the listening
position, but they were merely at more pleasing / flattering frequencies.

those reflections depend also on where the loudspeaker is and where the
listening position is

Absolutely. But in a small room there are NO locations that have a flat
response. Again, some places may sound better than others, but NONE of them
are accurate.

my main speakers are actually really good in the bass range, but their
bass range gets seriously screwed up by the room acoustics below 100 Hz,
and, as I said, I can't move them and/or the listening position

If you understand that it's a room problem, why do you resist the idea of
fixing the room?

You are welcome to keep trying to fix an acoustic problem with gear, but in
my opinion it really is the wrong approach.

--Ethan