The Petrucci tone, regardless of what amp he plays, is really all in his fingers and technique. He would sound great through a Crate modelling amp. However, this said, I would have to recommend a Mark IV. I owned one for five years, and I came as close as I ever was to attaining that sound. The lead channel is heaven, and the clean-slightly broken up sounds were there. There is a lot that goes into an amp decision such as this...do you play in a band, how big are the venues you're playing, what effects do you use, and what kind of guitars you play. After a few years of gigging, I would recommend a combo, unless of course you have roadies and you're playing huge venues. The Mark IV combos are fairly easy to come by, albeit expensive. If I were you, I would look for a used one, and with the money saved, buy a nice rack mount processor, such as the TC Electronics G-Major. You really need to nail that delay effect to get his tone.