Running DCS Vivladi DIRECT?

It could simply be that a tube preamp sounds sweeter and more polished and smooth and therefore is preferred by folkfreak to the Vivaldi directly. In this case, both George and folkfreak are right for different reasons. The Vivaldi output has nothing limiting about it (as measurements suggest) but the sweeter tube sound is preferred for system synergy and listener preference....

The Vivaldi driving a 600 ohm load is commendable, no denying that fact. The VTL MB 450 has an input impedance of 42K. So the question is which option provides the best sound quality

This is where things fall apart, the "best" to me is like a piece of wire, the most transparent/dynamic/least coloured/distortion.
So how can a preamp provide the best sound "quality" if it colours the sound? What it's doing is providing a colouration (which is distortion) that the listener prefers, but! all preamps sounds different, so which one with which colouration/distortion do you pick?
Wouldn't it be better not to do a band-aid fix and to  get the source sound "right" and forget about trying to change it with a preamp.?

Cheers George   

The input sensitivity of the VTL MB 450-iii is a function of damping factor but at the setting I use (minimum damping) is a low 775mV hence one needs to be very careful in setting the level on the Vivaldi. Luckily dCS provide a 0.6V (and a 0.2V) setting as well as 2V and 6V.

I currently use 0.6V with my active pre-amp and find that this works well with the volumes I like to listen at (i.e. operating the amp at close to unity gain in effect) so my presumption is this would be the best place to start with a direct connection so as to minimize the amount of cut I need to take in the DAC volume control and more importantly avoid the danger of over driving the speakers with catastrophic effect (this has happened to me in the past when I had a cable short and it blew the woofers in a set of Magico V3s)

Driving the DAC direct to amp has coloration as well. Use a half dozen different DACs to drive an amplifier and you’ll have 6 different sonic characters, they are not straight wire with gain. Common sense approach is what folkfreak is going to do, listen to both in his system and make a choice. ARC REF 40 coloration/signature or Vivaldi coloration/signature . You have to hear something before you can choose a preference. 
Charles 

Use a half dozen different DACs to drive an amplifier and you'll have 6 different sonic characters, they are not straight wire with gain.

Correct, why pile one on top of the other, just get the right one to start with..

I need to take in the DAC volume control and more importantly avoid the danger of over driving the speakers

And also just as important is not too start "bit stripping" if the volume is too low.
Also it would be interesting to find out if the ring dac chip in the Vivaldi is single ended output or balanced, if single ended then to make a xlr output they would need an extra opamp in the signal path, which would mean the se output should sound better. But if the ring dac chip is balanced then the the opostie will be true..

Cheers George