It's been shown time and time again that the presence of a single 12ax7 tube in an otherwise completely silicon-based circuit hardly introduces what some consider "warmth".
Yet I digress...if you think that it adds "warmth", so be it. 
For the most part, I think "warmth" is a buzz-word for inaudible distortion (to the regular ear).
So, slapping a tube in the path makes a difference only depending on how it's driven, where it is in the path, etc.
Just like a tube mic pre. I like the one I have, but it's really not noticeable until you crank the gain to just under the breakup point of the tube... and then who's to say that "warmth" isn't being added by the loading of the numerous transistors in that circuit as well?
I have a FET-based booster that I love, it adds warmth...
It's all based on application and execution, really (and about 85% perception to be honest--and this is coming from a tube guy lol).
Really, putting a tube pre in front of a digital effects engine... kinda silly. "Warm" up the signal to then tear it to binary shreds, process it, and then piece it back together as an analog signal before feeding it to your amp? That doesn't add up in my tone equation.
However, feed your signal through a digital processor, then to a tube power amp... I could maybe go to bat for that.
Maybe
