Carlos,

It's large but squareish (4.6 x 5.5 x 2.5 m)

As far as room acoustics is concerned that's a small room, and so it suffers
from the same problems of skewed low frequency response that are common to
all small rooms.

there's always a big valley at the listening position, so I think the best
solution is the sub.

A big dip in the bass range in a room that size is ALWAYS caused by acoutic
interference due to reflections off the walls, floor, and ceiling. It might
be fun to build a sub, but it's the wrong approach. (Unless your main
loudspeakers really are deficient in the bass range.) Not only will a
subwoofer not solve the real problem, it will likely make things even worse.
However, it will give you more "thump and boom" if that's all you care
about.

Have a look at my Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

It's not what you asked for, but I believe it's what you need.

--Ethan