While I tend to agree with you, there are no absolutes in audio. It may be more of an integration or room issue than the sub itself. I am using the high-level inputs as well, fed from Vidar. It could simply be that it is slightly out of phase with the speakers. Unfortunately that sub only has a 0 or 180 phase switch, nothing in between so I am left to moving it around and other adjustments to try to integrate it better.
The other possibility is however is that this sub is just not big or powerful enough down where I am trying to get it to operate, which is below ~40Hz. It is rated -6dB at 30Hz, so just might need something bigger unfortunately.
But we'll see. Need to do more experimentation.
Edit: also don't get me wrong. I find the bass to be very good, just a slight bit more sluggish sounding than the speakers is all.