Let me hijack the thread back to the subject!! LOL
I've often wondered how much of the actual feel and tone is related to the tube in Valvetronix amps. I'm assuming it's purpose is to create a digital modeling pre, fed to a mini tube section that warms everything up and adds touch responsiveness, then it finally gets amplified by the transistor solid state section. I think the amp sounds bigger with each click on the wattage control, but you can add a more compressed feel by turning down the wattage, and cranking the master. When you're in that mode, spin the amp dial, and see which amps really benefit from that.
Another cool hidden feature is that the volume for the headphone also controls the recording out, so you can dial it nicely with the trim pot on a mixing board.