I only have 100 watts at 4 ohm as of now, potentially 100 x 2 at 4 ohms.
No a life and death issue with most subs. Just beware that your amp is power limited, so don't blast bass heavy music too loud. Consider setting gain with DMM.
Now that you mention you have only 100watts, this got me thinking that you will be served by an unconventional subwoofer. Most subwoofers on the market are now the so called "long throw" subwoofers, the kind that have long voice coil travel and high power handling, but they don't always have great sensitivity. In the old days of home audio, the subwoofers did not have long throw or high power handling, but whatever throw they had was accessible with a weak amplifier due to a very good sensitivity. So I am thinking to suggest something along the way of the "Dayton Classic" subwoofers sold by Parts Express, or maybe even Dayton Classic woofers (they do have 10 inch classic woofer, which is different from Classic Sub).
http://www.parts-express.com/pe/showdetl.cfm?partnumber=295-202
The 10 inch Classic subwoofer has only 200watt sensitivity and 8.3 xmax travel, but the sensitivity, as measured at 1watt/1meter, is 89dB, which is impressive. A typical car 12 inch car audio subwoofer has 85-87dB sensitivity as measured at 1watt/1meter. (Don't trust the higher sensitivity numbers of car audio subwoofers. Most of them are measurements of SPL at 2.83V/1meter, so that's 2watts for subs wired for 4ohm, and 4 watts for subs wired for 2ohm, and 8 watts for subs wired for 1ohm, and they don't even tell you what method was used and how the sub was wired).
For a lot of music 100watts could be enough. I heard that a sub with typical music usually sees less than 100watts. By that I mean, something instrumental like the bass guitar or drum. For synthetic bass music, like in rap or dubstep, 100watts may sound less adequate.