i've done it a few times. it's actually pretty cool. the hartke has a different kind of od that reacts well with the vt's od. hard to describe the od in the vxl...it's more solid-state sounding but it's not what you'd call harsh. it's mid-scooped if you run it full-on, but the blend knob lets you get a goodly amount of clean amp tone in there without losing intensity from the od circuit, so it's easy to get mids back if you want them. the vt's overdrive reacts well with pretty much any od you can throw in front of it, but the vxl into the vt is especially cool since they're both so different. ymmv but i give that combination a thumbs up.