I own or have owned a number of Omega speakers and can share my experience. I'm in a 15' x 22' room (330 sq ft, or 30 sq meters) with high ceilings. Amplifiers are tube and solid state, ranging from 4 to 25 wpc.
Speakers in rotation:
1. Original Omega Super 3 (monitor, wide baffle, retrofitted with RS5 driver).
2. Omega Super 3U (traded in). This is a monitor that has the same cabinet volume as the Super 3XRS.
3. Omega Super 3HO.
4. Super Alnico Monitor.
I also own a pair of Rythmik F8 subwoofers that can cross over as high as 250hz.
I'll cut to the chase: a single-driver RS5 speaker will run out of steam in a room that size. I'm currently running my Super 3's in a near-corner location and while they sound great (better in some ways than the dual driver Super 3HO.) they quickly reach their dynamic limitations. The Super 3HO does not seem to have the same dynamic limitations in the space, nor does the Super Alnico Monitor.
The Super 3U has the same cabinet volume as the Super 3XRS, and I will assume that the speakers can work somewhat interchangeably.
1. If you want to use single-driver speakers (as in only one driver per cabinet) in a 25 sq meter room you will face some significant compromises. You can listen near field, at quieter volumes, or use bass supplementation.
2. The RS5- based speakers are really something special, but if you are trying to fill a large room you would probably be happier with the Super 3HO (dual driver) model. I encounter no dynamic limitations with that speaker in my largish space.
3. The Super 3XRS would really shine in a small space (like a 150 sq ft room).
4. The Super Alnico Monitor is a really big stand mounted speaker. It is closer in size to a JBL L100, Harbeth HL5+, or Yamaha NS690 than most modern monitor speakers. I say this because it is a rather more imposing speaker than the 3 series speakers. That being said, the larger driver works better in the large room and has a certain alnico magic that the 3 series does not.
If I were starting over I would follow this approach:
Get the Compact Alnico monitor. It is the same size as the Super 3HO Monitor (just turned sideways) and works in many of the same applications. I'm of the opinion that you will need bass augmentation in this large space, and using small monitors will allow for greatest flexibility. I don't think you lose much bass compared with the SAM, but you gain a huge amount of flexibility.
I would also commit to stereo subwoofers. I'm a big fan of Rhythmik for my needs, but in Europe you may find a better option. I've settled in on 8" paper cones for the bass modules to better blend with the Omega drivers. Thus far I have been successful.
I'm currently trying to focus two different systems in two rooms. The larger space is also my living room and needs to meet different needs, including entertaining and home theater. While we are not talking about large speakers, they can become overpowering when placed into a typical room.
In the living room/HT room, I am leaning toward the Super 3HO Monitors. These have really outstanding proportions and were an instant hit with my GF. They are out of rotation at the moment (hello SAM's!) but when they are they rest on an Isoacoustics stand, placed on a Rhythmik F8 with Herbies leveling feet attached. This is a really nice looking stack that is not very visually imposing. Other RS5-based speakers round out the HT array.
In the small room, I need to compare the SAM's with a couple difference Alnico Altec speakers (414/802 combo) as well as a pair of Alnico Goodmans Triaxiom 212's. If I can eliminate some of these drivers I can start to reclaim some of my space.