UPDATE 31/1/2022 I give it 4 stars, read at the end why...
Firstly, I would like to mention that the 310 model replaced the 300, as well as the 110 replaced the 100. My initial choice was to get the smaller model (110) at 160W, which, although it is available abroad, has not yet arrived in Greece. So, I had to either choose the previous small model (100) or go for the newer larger one (310). I saw and heard them side by side in the store, and despite the high price (529€) compared to 279€ for the 100, I ultimately decided to get the larger model (310). The main reason was because the speaker has a battery and you can take it outdoors without electricity, but when it's not plugged in, its performance decreases (it switches from performance mode to battery mode) and goes from 160W to 100W for the small model (100), while for the large model (310) it drops from 240W to 160W. My purpose was to be able to take it to the beach, so the small one wouldn't cover me for something significantly more powerful in terms of sound. Of course, the 310 is considerably larger and heavier (17.5kg), but with a little care, it can be transported just fine. I initially tested it in a closed space at home (in performance mode, meaning plugged in), and even with the extra bass OFF (it has 3 levels, OFF, 1, 2), everything was vibrating. The volume was at 30-40% so as not to disturb the neighbors in the afternoon. The sound was electrifying and the bass was incredible. I was used to the Samsung/Harman Kardon soundbar from the Q series, which is top-notch (700€), and the difference is noticeable, with the JBL delivering a superior sound. Then I tried it outdoors, on the beach, which is a completely open space and the sound spreads everywhere. Unfortunately, without being plugged in, it performed at 160W, but it still left me amazed as it was very loud, and within a radius of a few meters, 8-10 people could hear the sound perfectly and loudly. On the beach, I had it at 100% volume and BASS 2, so the battery didn't last as long as the manufacturer claims, but it's too early to judge that. I read that you can trick the speaker and put it in performance mode even outside by using a power station (a power bank with a socket input). However, that would be an additional investment of €80-€130. The front lights synchronize perfectly with the music rhythm, and everything can be controlled through the very nice and well-designed mobile application. I haven't tried it for Karaoke yet, but if I do, I will update. Generally, €530 is a bit too much, but unfortunately, there is no competition at this price range for a battery-powered speaker with this sound quality. I see in the history that it has been close to €400. At that price, I would say, if you don't mind the size, go for it without hesitation. Finally, I would like to note that the build quality is not just premium, but super premium. All the materials are flawless and properly finished. The speaker is waterproof (IP4) and fully sealed, but I wouldn't risk sitting and listening to music in heavy rain, maybe in a light summer shower.
UPDATE 1: After some time of use, I continue to be fully impressed with the sound and overall quality, but portability is a small issue. It barely fits in the trunk of my car (sports coupe), so I have to place it in the back seat, taking up a human seat. There are days when I don't take it with me outside, while if I had the smaller 100 or 110, it would definitely be much more convenient and I would have it with me all the time on my outings. However, I still believe that I made a good choice since the 110 is not yet available in Greece. I have to say that the battery is definitely not what the manufacturer claims, it lasts about half the time. I've also heard this from other partybox owners that I randomly met. However, I bought a power station from a Chinese website (iMars PS-S70) and I basically have it plugged in and outside at the beach, and besides playing better in terms of quality/bass/volume (performance mode), it allows me to have it on for a much longer duration (running time). I still don't know the total running time, but I use it for hours and the power station doesn't feel it, it has never dropped below 50%. It was worth the €120 that I paid if you really want to use it all day at the beach.
UPDATE 2 (31/1/2022): After several months of use at home now (due to winter), there are many complaints from the neighbors, not about the volume but about the bass, which continues to vibrate everything even when it's turned off. I'll give you an example: we were in an Airbnb house and we were playing quite loudly with the bass boost off (turned off), and people from the opposite apartment building came to complain that it was vibrating. Unfortunately, there is no way to turn it off from the speaker itself, only through the application like Spotify, if it has an equalizer option. If you listen to a lot of YouTube like me, you are doomed to listen to it quietly because from a certain point onwards, you hear windows, glasses, and plates resonating. It's a shame that there is no option to lower the bass, it's a feature that would be discouraging for me to recommend it to someone. I have it and I don't use it often because I simply can't. For this reason, I'm lowering my rating to 4 stars.
UPDATE 3 (14/3/2022): After contacting JBL, they told me that there is no option to turn off the bass, only if the application, like Spotify, has its own equalizer. If they receive many such comments, they might consider adding it to their own application, but I still see it as unlikely. I guess it will be included in the next hardware update, like the 320.