+1 on porting the HST-18.
Looks like you have a $1k budget. Buy another HST-18 and PA460 and port the HST-18s into the mid teens - full marty size.
You're going to loose a couple hundred on the resell of the HST-18, so you might as well work with it. LLT porting is going to give so much more output than sealed. You will probably see around 15-20dB increase in lower LFE going from a single sealed HST-18 to dual LLT HST-18s. All for about $1k. Use your current sealed cab for the 2nd PA460.
You might think you will loose output below tune, but it will still be more than a single sealed sub. The only difference is driver excursion. Depending on the amp you buy, you could easily get away with a 10-12hz LPF and not worry about exceeding xmax/xmech.
Theres hardly any content below 10hz anyway, and even the content below 10hz, isn't going to be played with huge authority (output) from a single under-amped sealed HST-18.
about how his transition from sealed to LLT has gone.
EDIT: I suppose throw away this post since I just read yours - I guess
If you want more 40hz output with less distortion, port the PS460s and tune around 30-35hz and apply a HPF with a DSP amp or miniDSP. Don't use a LPF on the HST-18s.