This is an interesting choice. I wouldn't worry about the 'musicality' of a sealed sub though, it's no more musical than a ported sub, at least if it is well-executed. Anyway, with the Hsu subs, you can always seal the ports if you decide you don't like the ports' contributions. As far as midbass goes, at 40 Hz and above I don't think you will see a big difference. Any of these option will have more output than you will ever use. The Ultimax will have a bit less distortion, but the Hsu subs don't really have much distortion in that range. The Ultimax does stay cleaner than the ULS-15 in deep bass, but if you push either hard, they do run into significant quantities of distortion in deep bass. The VTF15h mk2 run in 1 port mode, is very close to both distortion quantities and output headroom to an Ultimax, at least when I look at the Umax data from data-bass.com- but that was using a very powerful amplifier.
When I look closely at the performance data, I would say this: If you are interested in the music range, 40Hz+, it doesn't matter what you get, they will all be on par with each other. From 16 Hz to the mid 30s Hz range, a VTF15h in ported modes may have a slight output advantage. Below 16 Hz, the Ultimax should have a very significant advantage. Keep in mind that the Ultimax will be a bit more power hungry than the Hsu subs for the same SPL. I think the VTF-3 mk5 will have performance very similar to the VTF15h above 25 Hz or so.
