Great purely for preamps, horrible for full on modelers
First I'd like to preface this review by saying that the PE60 is fantastic at what it was originally designed for - as an extension cabinet for their Trademark series of amps that feature multiple preamp types going into a clean cabinet to feature the wide array of tones that the Trademark is known for. Somewhere along the line, Tech 21's marketing department decided that they should shift their focus from that to making it appear as if it's a great solution for amp modelers from every manufacturer in the world. Take that with a grain of salt. If you have ever played a POD, GT, RP, Eleven Rack, Amplitube, Guitar Rig, etc. through a true FRFR configuration (PA speakers, floor wedges, bi-amped studio monitors), then you know you are getting the ultimate representation of your gear's tonal range and flexibility.
The Power Engine 60 sits uncomfortably in the middle ground. It's not nearly as nasty as running full modeling (models of guitar amp, speaker cabinet, mic, effects, etc) through a traditional guitar amp with pre and power amps coloring your sound, but it's not anywhere near high fidelity as running through FRFR. It sounds like someone put a thick blanket over your sound, and no amount of tweaking will get you where you want to be. You will be stuck with an awkward compromise which is neither here or there.
That being said, if you've been struggling with running a modeler through a traditional guitar amp and you want the look of a traditional guitar amp on stage with you, then the PE60 will sound worlds better than what you're used to and will certainly fit the bill. However, if you're not concerned about people asking questions as to why you want to plug straight into the PA, or having your own personal active PA speaker, or keyboard amp on stage as opposed to a real guitar amp, then kudos to you for breaking the status quo.