Analog challenged by room, space and esthetics.

WOW!   A pair of No.25's AND the rest.

That certainly isn't an entry level setup. 

Room aesthetic is a big factor for me also, since I don't have a dedicated room.

For that reason, and simple ear tuning put both subs(old REL B3's)  behind the listening sofa  near the corners of the room roughly equidistant to the main speakers in the front. Acceptable SQ  without having to  see more unsightly boxes cluttering the view. Probably a mess if it were graphed via mic/software.

I bet it sounds great regardless of the bass concerns.

You want the miniDSP 2x4 HD.  However I have to tell you that the sound quality is going to be entirely on your set up.  Don't go for flat.  Go for the subs to descend about 1.25 to 1.5 db/Octave from 16 Hz down to 80.

Consider as a minimum GIK corner traps from floor to ceiling in the front corners. 

If you won't do that, then your best bet is to focus on what you can clip, and getting the tilt right while ignoring nulls.

Lastly, measure the frequency response from 3 places at least.

@erik_squires I Listened the MiniDsp 2x4 and the Flex, and to my ears was too restricted,  and definitely rather go without it that using them.

I was hoping for something more transparent than either one.

I don’t want to add more sound treatment, the house and living room are already overwhelmed with the audio system, and it has to feel good for the both of us.

The other listing rooms in my actual home in Barcelona have more latitude because they are bigger.

@tablejockey thanks, my father loves music, and he game an tube amplifier to restore the I was 9, if I remember correctly. I am autistic and that project helped my father to have something to do with me and gave me something to do. All my life I have collected audio systems, and in 1988 my father introduced me to Mr. Hiroyasu Kondo-san and 2 or so years later I bought my first serious amplifier from him that I still have and is my most treasured amplifier, and since then I have been fascinated  by how good music can be in a room.

@astolfor  Wait, did you use them for just the subwoofer (like me) or in the main path?

You might want to try adding a third sub which will go a long way toward smoothing out the room modes.It doesn't have to be the same brand. There's no risk if it's returnable or you can borrow one for a weekend.

@erik_squires we connected his to my subs and spent some time measuring. The 2x4HD and the Flex made a difference, but at the expense of openness, dynamics and precision(?). I am sorry English is not my strong language.

@jtcf I agree but the living room is not very large to put more subs or add more sound treatment.