I found most of that SVT magic is in the power amp. If you have a hot bass, plug it directly into the effects return (if it has one) and you'll hear what I'm talking about. I owned a few different Ampeg preamps with high hopes and it never had that sponge magic that we all really dig. That 300 watt all tube power amp is really incredible and is what separates it from the rest.
On a different note:
Nobody talks about those Tech21 heads. I've found a happy, reliable medium there. It's dead quiet and really satisfies that quest. The Landmark 600 is 40lbs. and I can deal with that if it gets me there. It's reliable, quiet, and has a great D.I.
I've never met a sound man that doesn't dig the Sansamp. A lot of them scratch their heads wondering why bass players just don't go with that.