you can go really small with the lms, but the tradeoff is that you won't be able to put enough power on it to get it to xmax. small boxes are nice, but there is a point where the sensitivity is so low that the driver can't take the power required to push it to xmax.
both the 18ho and the um15 model similarly with the 18 being a little more sensitive on the top end. the um might be a little better controlled for critical listening on the top end, but who knows without a side by side.
the stereo integrity 18's are good for ht too.
a quad set of dayton 18ho's in 3.5 cubic feet of enclosure per driver and a 5000 watt amplifier (cerwin vega cv5000) and some sort of eq device would make for a meaningful upgrade over what you have now. the drivers are 4 ohm nominal, so 2 in parallel per channel pull full power from the amp.
the -ho has been tested at data-bass.com if you want to see performance.
good luck with your build.
here is 5000 watts total power on either:
1. 4x dayton 18ho's 3.5 cubic feet per driver, and
2. 1 lmsu 18 2 cubic feet