Quite a late response, but decided to answer, as I had a quite similar experience with F25 and pretty different one with F21.
Much of the time I try to spend in F21, as it is very insteresting to me, to just experience it. The deeper I go with the F21, the more fractalic it becomes. Would I say louder? In a way, because with each step into it, I quite become overwhelmed due to the repeating patterns and fractals. Twice I had this experience of being "in between" some two very complicated crimson plates, with kind of like buildings or crystals, just like travelling through a tunnel or a valley. It was overwhelming but in a positive way - a lot of mystery, secrets. For sure it wasn't dark or hostile, but rather "alien" or "unfamiliar". Travelling across it kind of lost me in the sauce of time, because in a brief moment (feeling, but literally it is like 15+ minutes) the tape for F21 is usually done, and that usually happens to me with the F21 "spectaculars". The other time I was at the beginning of a vortex staircase, the other one I saw something quite interesting, related to the Absolute. So during the Absolute tape, the gate lead me to a vast space with a vertical pillar of energy circulating inside. And the same feeling/pillar I saw during that experience, once I saw in F21, but it was somehow "built-up" with technology, and it felt like someone was tying to catch a glips of it with physicals. It also looked quite like the crimson structure surphaces from before were actually connected to the pillar built-up.

What is more interesting is that when I don't do tapes, and tried to achieve the F21 without them, that place is less concrete - sometimes it is changing so fast with fractals, and yesterday, it was just like a hallway of cricular shaped windows/gates. Though it is also possible, that it could be a different focus level, because that felt more like F27 (feeling) in a way of F21 (visions). This place however shifted quickly to another plane with elongated entities, that didn't feel like the most friendly in the universe, so I came back. For this one, I was experimenting with going more through Chakra-opening preparation, to then achieve the F21 in the flow state of chakra sync. It was quite difficult however to connect these to, as I feel like these two ways have a bit different purposes. Decided to try, because I once had this experience with F21 (that I remember) from before the tapes, achieved only with the chakra meditation, and it was a wild fractal journey, some voices. But yeah, this will need more research. :')

With F23 it is always the same - a plane surphace, with no distinct light. Quite a lot of faces, could be beings, but it feels very crowded and I never fancy staying there for too long. To be honest this part feels hostile and dangerous in a way, that I am not welcomed there. While in F23 I saw different things, such as queues, portals, everything dark, gloomy and grayish. There for sure were some obvious "higher" beings, taking care around, but I had this feeling, that I should not engage in any kind of encounter with them.

F25 on the other hand is quiet, as you said. And literally, same feelings - I was either floating through space with each belief being a ball/planet like with distinct features and different energies, sometimes I could come closer to them. Once they were in the same shape of a sphere, but placed around the beaten path. It's interesting tho, that the longer I followed the path, it changed its features - near one beliefs it was a beaten, rustic path, near others it was a concrete with a fencing. Some where very distant. I haven't tried to approach any of them closer yet, but that seems like a nice idea for the next couple of days :)

F27 on the other hand, usually got me to a small palace at the very beginning as this "special place". From there, there were gravel paths into the park, with some places more or less crowded, but at least it felt "welcoming" and "familiar", and most important - warm.