I guess we are talking about the same SACD.I got a SACD with the supposed original Steven Wilson Master.
It sounds at a lower level than the DVD. I tried to equate the volume levels and did some A-B compare, I noticed that the SACD is a little bit less boomy than the Dolby DVD and sounds more clear.
Specially I like more the high frequencies at the percussion and drum cymbals. And perhaps I feel the vocals more clear.
Not a big difference for my somehow old ears, but I think I can still notice a difference.
The one I have, guess what... has full bandwidth LFE... drums and bass isolated. Check it out.
This is a quite typical signature of Steve's surround mixes, I also compared the front channels and are obviously the same source as the ac3.
The SACD is indeed at a significantly lower level, but considering the bit depth of 24 bit vs 16 bit of the ac3 the SACD delivers for sure >100 times more amplitude samples. However that should be addressed for a proper HD release.
In comparison, the ac3 sounds... lossy to me.
The backing vocals for instance, come through in the SACD more clearly. The ac3 feels somewhat edgy.
Having said that, before I heard the SACD I was surprised that a ac3 mix could be OK and I used to listen to it quite frequently.
By the way, the SACD didn't include the nice bonus track "Frozen rose".