Lower frequencies travel easier through walls. The 8s go as low as 35Hz, where the 6s are stuck at 38Hz. However, this shouldn’t be too big of a difference. Especially when playing at the same volume.
The bass and high frequency level controls are really only for fine-tuning. I think +/- 2dB. Not really ment to change the sound color, but more to adjust for specific anomalies in the room acoustics.
If you plan on doing production too, the best way to setup your speakers is in a triangle with equal leg lengths, with your head being the tip of the triangle. So the distance from your head to the speakers and between the speakers is the same. This means the speakers are usually pretty close to your face (ears). Personally I think that 8s are just way too much in your face in that setup. It’s like you are looking at a wall of speakers 😀 which might be what you are aiming for.
I always say that the 5s (which I have) are a tad low on bass, especially for more EDM and bass-heavy music styles. The 6s in my opinion fare a lot better in that respect. If bass (or lack thereof) is an issue with 6s, I’d rather expand by getting KRK subwoofer with it than going for 8s.
As far as neighbors are concerned, be sure to start a conversation with them. Tell them what you do, why you like doing it and that you have to practice/produce to get ahead in the game. Also tell them that you want to try to limit any inconvenience to them as much as possible. Ask them what would be times that they feel would be least disturbing for them. Tell them to come and tell you when it’s annoying them for some reason. Show empathy and understanding and hopefully you will get some of that back.
Just my two cents. Hope it helps some.
Greetinx.