I asked how loud you listen...
Some people turn up the Bass with a house curve in their room correction for example... so while you may listen at -10dB, your bass could still be +10dB...
So how loud you listen is maybe more appropriately phrased as how hard you push the Subs.
As for a semi-sealed room... it's either sealed or not.
Mind, you don't have to do as some say and "pressurize" the room. However, LF Soundwaves are very long and pretty much do what they want... so in some ways, if your room isn't closed off, they will act on the bigger space.
That said, I doubt you really need to go crazy with giant Subs... You will be pretty close to them and the "semi-sealed" nature of the room won't matter as much.