i have a pq3 and a pq4, but my pq4 came broke and i never fixed it. i gotta get to that. the pq3 is awsome though, i used the boost settings from a thread here a few weeks ago and i never been more happy with my tone. it works sweet in the loop too
The difference is that the PQ3 is an EQ and a boost all in one whereas most parametric EQ's are an EQ only. The PQ3's boost feature can be used to boost your signal by +26db and you can use it before your amp to boost the signal with any freequencies that you might want to use (think of it as a flexable boost where you get to pick the freequencies)