My own vote would be to consider something like the B&C 18TBW100. It has a substantial efficiency advantage, lower inductance than typical HT subs, and sufficient excursion to produce significant output at 20hz. It would roughly equal an HT-18 at 20hz in the same sized box (5.5 cubic feet for example). However between 35hz and 100+hz it would have substantially more output with substantially lower distortion. I believe that B&C driver is about $350, and if you ever blow it, you can recone it for much cheaper than its new price.
There is also a new B&C 18DS115 coming that will have a lot more xmax and probably be a better option. I am hopeful it is priced similar to the TBW given the price of the other options, but it is not yet for sale from any of the known channels right now. I anticipate it being priced around $380, but that might be wishful thinking.
Another good pro option for cheap is the Dayton PA465S-8. It has higher inductance than the B&C and less XMECH, but its XMAX is very similar and it shares the same high power handling. Basically all of these options model at about 115db at 20hz. The difference is that where the SI-HT18 would still max out at about 115db's at 80hz, the pro drivers would both be at about 125db's. The Dayton really wants a bigger box though, it would do better in something that is more like 8-10 cubic feet and while you can tune it to 20hz, it would stay linear over a wider range if it was tuned to 25hz.
Data-Bass tests on similar drivers suggest all of them (the SI included) will max out a bit higher than the models suggest and in a room you can expect to see even more output due to typical room gain and boundary placement.