Well, if you are planning to get rid of the VS 1680, you would be able to use the Presonus Blue Tube pres with the Digi, also. Then you can make your own comparisons.
The VS1680 has 20-bit AD/DA converters, while the Digi has 24-bit. So, if I were to hazard a guess, I would say that you would get somewhat better quality with the Digi 001, than with the Blue Tube/VS1680 combo. But, what you really get with the Digi 001 is the Pro Tools software. This software is very simple to use, and very powerful with the 001. You get 8 analog ins and out, plus 2 SPDIF, which is about equivalent to the Delta 1010, except that you also get 2 good quality pre with channels 1-2. The pro Tools software lets you route these in and outs and edit the sound in very powerful ways.
But wait!! There's more!! You also get 8 more Adat channels in and out. And these can also be routed like the analog channels. But to use these, you will need an optical interface, ADAT recorder, or whatever. But at least it is there and make the unit very expandable and able to operate with the highest quality recording hardware available. So with the Digi, you get the capability of 18 simultaneously ins and outs. I guess it would be kinda like a Delta 1818 with two added preamps and a free copy of Sonar.