The Vivaldi volume control is digital one, you cannot switch it off or bypass it. Therefore no matter you connect the line preamplifier or not - you would always use its analog output section. If you find that preamplifier makes the Vivaldi sound better, this is because it introduces its own sound to the system usually by limiting high frequency, rolling off the spectrum above hearing capabilities.
The analog output section of Vivaldi seems to be top quality, I can hardly imagine it could be any better.
Any preamplifier would introduce something of its own to the sound, some may like it more and some may like it less. I can understand that introducing the preamplifier can make the sound more "digestible" is some systems, my advice is however to use Vivaldi direct to gain unlimited transparency, unless my must use preamplifier because of turntable or other analog source. If your sound is too bright or not so musical, search a weak part in your system that is responsible for that but this is not Vivaldi DAC, but rather a source component feeding it with the data or some digital jitter.
Changing cables may help a lot. I have found that putting some vibration control device under Vivaldi makes the sound much, much better. I would also recommend switching the Vivaldi buffer off, turn to 6V and use higher filter. I like the MAP 1 or MAP 3 better than MAP2. Also using power conditioner can help reducing a lot of "digital-like artifacts" from the sound helping to get more analog sound - check Shunyata Denali or Triton 3.
