Small Room... Big Stereo

What I gather from a review of your speakers is that the low frequency driver is crossed over @ 150Hz (no slope mentioned) and it's compartmentalized and rear ported).

Not certain if you have one or two REL's, but regardless it would seem that you have a "sort of" subwoofer "swarm" taking place (with two of the subs being rear ported).

Aside from the square room (really bad) it may sound OK if you can place the speakers in a diagonal placement (room corner between the speakers) as already mentioned  by russ69.

If not I would just pop for a pair of nice standmounts (like the new Harbeth P3 - ideally placed on the diagonal) and blend in the REL(s).

DeKay

Great system. I assume the general placement is governed by the doors and the room parameters. Assume speakers go on from the front wall out by a couple feet and in from the side wall by a three feet as a starting placement. A triangle puts your seat out six - seven feet from speakers. Then work on the room.

The room sounds like a challenge. I would work on it before the equipment arrives. But if it sounds like an echo chamber I would not delay. If you can, complete carpeting. If not, area rugs covering floor from speakers to listening area. I use hanging Afghan rug (see my profile for system photos) on front wall, for a small room like yours, on rear wall as well. Or you can use heavy wall hanging or real acoustic panels, but these must be ordered and carefully placed. Then some kind of deadening material at wall reflections (to listening seat) on your left and right (tall rectangular framed acoustic panels. I would treat the room until all obvious echos are gone.

That should be the basics. Then you can work on any specific issues that remain like ceiling. Maybe Tube Traps in both front corners. If area rug bolsters where front wall hits floor. Small corner (little triangular) acoustic dampeners.

Can't really say anything until you have the system set up and start experimenting with placement.  A square room is not ideal, but few non-purpose built rooms ever are.  The ceiling height will be a big factor.  Get things as good sounding as you can before applying external solutions.

Okay now your great gear in a sh-tty room. 
How long will you be in Munchen?

Might not be worth the RT fare on your gear
if you are in for a fixed period. Odds are something
will not arrive in the condition it left in.

Only out that comes to mind is if you never
listen above 69db.

Otherwise I would buy a Luxman 218 Tube INT
and some headphones.

I have Thiel CS3.7's in the same sized room. I took out the closet doors so I have a bit more space. I added some acoustics panels, I added an additional panel on my CD rack recently, and I also have incorporated professionally created Convolution filters (DSP) for my ROON Server (so digital streaming only).  I also keep my room door open.

This setup was great with the big speaker in the tiny room. However, recently during some equipment demos I accidently realized that I had improved my room acoustics (the CD rack acoustic panel) and I no longer need the Convolution DSP. 

I have some old photos of my room on the virtual systems. The difference today is that the CD rack is covered with a panel. I have also moved a large computer/TV monitor much further away from the first reflection point of the LEFT speaker.

This guy can remotely create Convulsion filters  specifically for your room and speakers.

https://accuratesound.ca