I have two that I really dig, and I go back and forth. MI Audio Crunch Box V2 and Radial Plexitube. I haven't changed the tube in the PT yet, but I have a good one sitting here ready. Right now I'm going with the Crunchbox into an AC30 through greenbacks, and getting great Marshally tones. Turn off the pedal and instant Vox cleans, really the best of both worlds for me right now. Of course, it sounds best when the volume goes up.
The plexitube is great, better at low volumes, much more distorsion than the crunchbox, better overall tone control as well. I'm still learning about it and having a great time. Right now it's become a killer living room tool, plugged into a traynor bassmate SS bass amp set to super clean. Fun!