15" and less that 20kg is a rare combination. A Yamaha DXR15 gets close with 49Hz, 15" and 22.5kg.
I would recomend the EV / ElectroVoice ZLX 15P (17,3 kg) like I wrote in this Thread
Yamaha DBR15 or DXR15 would also be good candidates and rather a matter of taste. In my comparison, however, I preferred the ZLX.
"I tested PA-boxes myself a while ago to amplify my bass/kemper. I would definitely recommend using a 15" speaker, because 10/12 lacks a lot of low end. I was surprised how much is missing while testing it in the musicstore myself. Even if I compared expensive speakers like the QSC 10/12.
I finally ordered the Yamaha DBR15, EV ZLX 15, JBL EON 15 and tested them again with the band. (in the Musicstore I also tested the db technology Opera 15, which scored with incredible bass, but overall it was very undifferentiated)
Here I give you my short review, that I wrote in a german forum. I translated it quickly through the deepl.com-translator, hope it is understandable so far...
"... At home I had to realize directly that the JBL EON is completely unsuitable in the floor monitor position, because it tips over at the slightest touch. The ZLX and DBR, on the other hand, sit well on the floor.
During the apartment test at an acceptable volume, the DBR sounded tidy & differentiated compared to the others, but had much too loud highs and the midrange faded too far into the background. EON & ZLX were about equal up and quite balanced, albeit somewhat worse resolved. I was still missing some bass at the EON.
With the DSP I was ahead after the ZLX, because I got the best results here and you can still adjust a lot. (I only changed the EON DSP at the box, via Bluetooth you have even more extensive possibilities, but...) the EON dropped out as floor monitor and the missing added value compared to the ZLX and the DBR and ZLX were dragged into the rehearsal room on the 5th floor. To mention here, the ZLX is easier to carry, has more handles and 2kg less on the scales.
So, now we could start under the actual test conditions. First, loud music is played and both speakers are compared, again, DBR sounds tidier but a bit shrill. The ZLX sounded too bassy at first, but the DSP let it fix directly. ZLX now sounds good, but not so differentiated and a bit more compact in sound. Our critical guitarist was on the side of the more tidy DBR.
Next step was to play some songs with the band and to light the monitors with my Kemper bass signal. Both PA-Boxes about ~90% turned up (before the sound just suffers or the limiter starts) and played alternately. I would say that the Yamaha can tack one louder than the ZLX, maybe the ZLX also has some air upwards, because the limiter didn't even have to intervene at the set volume (very nice to see via the front LED, super feature)
Then it started, who will prevail against 2 guitars and drums (Metal):
The DBR had good bass pressure, but where did the shrill highs go? They were completely gone in the band mix, no more audible/capable treble, I couldn't hear my touch/attack sound, mids were only locatable somewhere in the backround and that although the DBR was a bit louder than the ZLX.
ZLX tested, what can I say, it was a little quieter, but the bass sound came through the way I know it at home via Kemper on studio monitors + just band. You hear the amp character, bass is there, the touch/attack sound in the highs is present, it cut trough the mix, great
We played a few different songs, once DBR, once ZLX and our guitarist always spoke up, whether A or B pass = the winner was always ZLX. It was also clear to our guitarist that you would definitely take the ZLX. Great sound, money saved compared to the DBR and still 2kg less to carry, I'm happy on occasion will be ordered a second one after. ""
Honestly, you don't need frequencies below 50hz.
Even the legendary Ampeg 8x10 only works down to 80hz and I've never heard anyone say there is not enough bass
Frequencies below 50hz just buzz/rumble the stage live. What you really hear and where you locate your tone is in the 100-200hz range. It is more important that you have the appropriate sound pressure in this area.
I got the ZLX-12P first and liked them enough to get the ZLX-15P.
The fundamental of a low bass string is around 40 hz. At little bit of that is always nice
Yeah, I know where the frequencies are. I play 5 Strings with Low B or Low A, where the fundamental is at ~30,9Hz/27,5Hz.
IMHO I don't understand why the prejudice prevails that the fundamental frequency must be reproduced in the bass. What we hear 95% are the higher octaves of the tone. And if there's no high-quality subwoofer in the setup, we feel just vibrations/hum or it's not reproduced by the speakers at all. For example, you can test quite well by reproducing sine tones below 50hz on your system.... corresponding subwoofer presumed
I mean, anyone can do whatever they want, but I don't think it makes much sense. Save the money, just so the speaker can get 5-10hz lower, ect
Try to lowcut your sound testwise at +/-50hz and in my experience it doesn't make any big difference. In most cases the sound often becomes clearer, more powerful and easier to locate, when the hum under ~50hz gets removed. The speaker is unburdened and these low frequencies are at least tonal not important.
In hifi or recording studios that's another thing, but in most cases this is rather counterproductive, especially on stage or in the rehearsal room.
The RCF are certainly well suited. But I haven't tested them yet.
Thomann recommended the RCF to me at that time also additionally I often positioned them as floor monitors and preferred monitors with a completely closed front grill , so the RCFs fell out of my selection.
In retrospect, I can say that the ZLX might be a little bit louder in some really loud situations before the limiter takes hold.
But with the compressor activated in the Kemper, the volume can be increased quite well without being limited. In the worst case scenario, my second ZLX15 will be dragged along to the gig. 
My Bass needs a baby FOH for my rig to maintain headroom and volume. I only play loud enough to balance with the drums.
I often positioned them as floor monitors and preferred monitors with a completely closed front grill , so the RCFs fell out of my selection.