Velodyne ULD-15 II sub performance

The ULD -15 sub only allows adjustment of the "subwoofer level" volume control. When listening to music at low volumes, the sub is invisible unless I turn the sub's amp to 80%. Is this typical of this sub woofer? I thought it is a 400 watt amp? Shouldn't it put out more sound and lower volume controls?
Congratulations on meeting Messrs Fletcher & Munson for the first time. It's not your woofer, it's your ears (Mine, too. His, too. Hers, too.)! Check this link for further explanation:
This is one of the handful of subs that I believe is truly made for music and not for chest thumping home theater effects. The bass is very smooth sounding which never muddies the high frequencies. Infact, it accentuates the highs during my listening experiences with this sub. Since this is a downward firing sub, you need a solid foundation for the sub to perform optimally. IF you living in an apartment or something where the floor is sort of hollow, then you might get reduction in volume.
Very interesting webpage Alrau1. I can relate to this as I always start with a sound pressure level meter to match all the speakers with test tones when I reposition the speakers. I do this at fairly high listening levels. And usually I find that I prefer the sub's level down just a little bit from the SPL meter's reading.

As DVDgreco has observed, I too notice there is not a lot of energy from my subwoofer (Velodyne ULD-18II) at low listening levels so I inevitably turn up the sub's level a little to compensate for this. And then when the music gets so loud, I turn the sub's level back down .... and ultimately (from then on) it pretty much stays where it was when I set all the levels at the start. I find that I am less demanding of bass energy at low volume levels so this is fine. But it can end up being a forever game of adjusting the level on the ULD amp. And I find myself doing this a lot with the two ULD-12's in my HT systems as well.

Concerning the Velodyne ULD subwoofers, I must concur with Viggen on this. I have tried many subs for my audio and HT systems and the ULD-18 (crossover set at 50hz) I use with the Maggie 3.5s has worked very well. So many people say they have not had success with matching Maggies with subs but this has not been my experience with the ULD-18. However, now that I am driving the Maggies with Wolcott amps, I have less of a need to use the sub even though it still makes a difference on some music.