Just dropped the PA460 into the empty sealed cab. Set both subs to around 80 dB and did some sweeps. The PA460 is flat to just below 40 Hz. Just above 40 Hz, the HST gets a good sized null in its response up to 60. So combined the 460 flattens out the response very well. The 460 has a better response above 100 Hz, but this could be in part due to the subs being in different locations. I did not spend a lot of time with integration or eq, but it already sounds very very good on music with a lot of chest slam. Very impressive. Running no kind of high pass filter on the 460 as it held up fine with above reference level since wave sweeps.
If I find the time I'd like to run some more scientific tests, such as close mic'ing each driver to compare base responses, as well as testing them both in the same location or perhaps even swapping locations to see if response can be improved. Honestly I probably won't go through all that but I would like to at least swap locations, then spend some time improving things with distance/delay settings and a bit of eq. I'd like to see some improvement in the response above 80 Hz but I fear this is due to my seating location and the setup of the room, which would be very difficult to change due to openings to other rooms and the main entryway.
Regardless, the 460 integrated much better with the HST than I imagined. Their combined performance on movies and music is very impressive and I feel like the combo would be very tough to beat in my room. Still have a UM-18 on order and hopefully it will sound as good but will reinforce output below 40 Hz.
So, stick with this setup, or keep the UM-18 on order and compare? Decisions, decisions.