Preamp for Bel Canto DAC3.5

I currently have a Bel Canto DAC3.5VB MKII running direct into a Pass Labs XA30.5 and have been toying with the idea of putting a tube preamp between the two. I do like the sound of my current setup but have considered that adding some tubes may liven up the sound a bit.

I was wondering if anyone had experience any similar systems and would like to share their feedback.

And, this may be an absurd question, but is there a way I can add the tube sound without the inclusion of volume control or input selection? My DAC already covers these functions so, in an ideal world, I would just "tube" the signal. Is that crazy?

(If somebody goes on to build the "Tube It" and gets crazy rich, you- at the very least- owe me a free unit...)