I don't doubt that both the REL HT1205 and Rhythmik L12 are very good subs that work vey well in your systems. There are also other very good subs currently available that run the gambit from relative bargains to quite expensive.
However, the OP Hans has expressed an interest in considering the 4-sub Audio Kinesis DEBRA bass system with the four subs positioned around the perimeter of his office in a distributed bass array (DBA) arrangement. The 4-sub DBA concept is a new one to Hans and many others, which is why many of the recent posts on this thread have concerned the concept's explanation, its effectiveness and possible further refinements.
In the spirit of educating others on the DBA concept, both of you mentioning alternative very good reasonably priced subs that Hans could consider using raises a very good point and relates to other important advantages of using the 4-sub DBA concept: its flexibility and scalability. I believe anyone considering a DBA system should be aware of these for best results.
Its flexibility is demonstrated by the fact that effective custom DBA systems can be created using any subs an individual prefers. Its scalability is demonstrated by the fact that the actual in-room maximum bass power, impact and dynamics can be increased and decreased according to the subs utilized as well as adjusted by the setting of the volume and crossover frequency controls.
The essential main requirements for an effective DBA system are that four subs be used and that they're properly positioned in the room. There is a vendor offering an effective DBA system utilizing only three subs but it requires professional positioning of the subs based on a proprietary computer program.
The acceptable actual subs utilized in an effective custom DBA are less restrictive and can range from using small DIY subs containing 8" drivers, like clio09 uses, to using the largest and most expensive subs available.
I also wanted to make sure Hans understands the practical distinctions between installing and configuring a 4-sub custom DBA system versus a complete kit DBA system like the AK SWARM or DEBRA systems. The main method of seamlessly integrating a sub or subs into one's system are the proper setting of the Volume, Crossover Frequency and Phase controls.
With a custom DBA system, using traditional self-amplified subs, all three of these settings are required to be set individually on each of the four subs. With a complete kit SWARM or DEBRA DBA system, using passive subs amplified by a separate external amp/control unit, all three of these settings are only required to be set once for all four subs as a group on the amp/control unit.
Another option, if you're handy and up for it, is to buy the same sub amp/control unit the SWARM and DEBRA kits use and build four passive subs of your choice. This would allow the setting of controls once for all four subs but it could be either more or less expensive than the cost of the SWARM or DEBRA complete system, depending on the exact sub components chosen and the dollar value you designate for your own labor.
Tim