10 times the power gets you 10 more decibels, so since 100 watts is 10 x 10 x 1 watt, you should be +20 decibels.
This assumes perfect linearity, which you will not achieve, so your real world result will be less than this due to compression and losses. I don't know how much you could expect to lose, it would vary with the driver.
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.
Yet again the myth of room boundaries adding 3dB appears!!
Every boundary actually increases low frequency sound pressure level by 6dB, easily verified by measurement and theory. Just look up in Acoustical Engineering by Olsen, amongst other classic textbooks.
Eric, sorry to correct you so publicly. Re reading my post, I hope I didn't come over as rude, it was not my intention.
Aggemam, I did carefully qualify my statement by saying low frequencies, and boundary reinforcement is by definition reinforcement by a boundary, not a "near" boundary!
One has to be very careful on a forum such as this to make clear the conditions that apply to statements being made. They are open to misinterpretation due to either being inconcise when making statements, or being misinterpreted when read. At least it can lead to lively discussion!