I guess this shootout really shows the performance of the db K300 and FBT J5a speakers.

Directly compared with the Nano 300 near maximum volume, Nano 300 set to a decent subwoofer level of 25% but cranked up to the point where the green display turns orange at each drum kick. (The orange light would come up at much lower levels if the sub were turned up higher).
So it's a very fair comparison, each system is turned up to its maximum usable level.

I think for those who have used the Nano 300 themselves, this has given you a clear idea of how the small FBT speakers and the still quite small db K300 compete against it.

I'm quite sure using one of these together with a Nano 300 is equally powerful and more balanced than using two Nano 300. I'm just not sure whether a Nano 600 would be superior to this combination (except for the stronger subbass).

Sure appreciate your efforts especially in mini/portable PA systems. It is informative. I understand you are looking into the Nano600. I used to have two Nano 300s, but still own one besides a number of other amps/speakers. I don't know if you do a lot of vocals, but you want to make sure vocal sounds good through the Nano 600 because I didn't really care for how the Nano 300 handled vocals. To me, it was a bit muddy. I like vocals through my other systems much better. As subjective as sound is, I think it should be looked at from multiple angles.

For my part, I normally don't sing. But once we used my Nano 300 in our band project, and we weren't happy wih how the Nano 300 rendered our female singer's voice. Maybe we could have achieved more with an additional mixer with 3-band equalizer, which we didn't have at that gig.
But luckily I also had the FBT J5a at the gig, which I had first only connected to my synth, but when we connected both Nano 300 and FBT to the mic, the voice sounded much much fuller.
So I agree, there is something a bit muddy about the Nano 300 top speakers in some situations.