In my case I've been playing around with tubes in a preamp. I've been trying to sell the preamp for some time, but I figured I'd break it out and do a little experimenting as long as I've still got it.
While its not as optimal as listening straight out of the DAC, different pairs of tubes have pretty different sound signatures. Some of my older "tubey" sounding tubes sound pretty darn lush through the 9-HV combo.