SPLs of room interaction
Theoretically (which NEVER happens), each "close proximity" room boundary such as walls and floors will add 3dB to the sound level. This is why corner loading your subwoofer can add up to 9dB to response in the 20-40Hz region. However, reflections from walls and other objects will cause the sound wave to interact. The results of the interactions will nodes and antinodes, some will increase your SPL while others will decrease your SPls.
Have a look at the
Room Mode Calculator and plug in your room dimensions. The basic idea is that you don't want the room modes (given in Hz) to be too close to one another. You want to seem them fairly evenly spread out rather than be "clumped" together.
To more directly answer your question, each time you double your speaker input power, you add 3dB to the SPL:
If 1 watt at 1 meter = 90 dB, then
2 watts at 1 meter = 93 dB, and
4 watts at 1 meter = 96 dB... just keep adding 3 dB. The final results are, of course, subject to the limitations indicated above.
Hope this helps! Eric