The in-built amp in the GPX's is very good. It is extremely clean sounding, with no extra hiss/noise/distortion added. With a properly wired GME speaker, there is plenty of volume. I shouldn't be saying this as I sell the B&Z's, but hey it's my honest opinion.
However, where you may want to add an external booster is if you want to run a dual speaker set-up. Gives you a bit of extra control, and having the bypass switch on the B&Z is handy.
Also over the years running GPX training days, if you get more than 3 people attending so everyone is spread out, and particularly on a windy day, I just couldn't get the speaker loud enough so everyone could comfortably hear what I was trying to demo. Cranking the GPX's Target Volume beyond 16 just makes the unit very "on edge" and touchy. So I quickly started using external boosters on training days. A Dual speaker with a B&Z works very nicely in this scenario. After the usual "gear interrogation" by some of the trainees, I often had trouble trying to convince them that I generally prefer headphones
However, where you may want to add an external booster is if you want to run a dual speaker set-up. Gives you a bit of extra control, and having the bypass switch on the B&Z is handy.
Also over the years running GPX training days, if you get more than 3 people attending so everyone is spread out, and particularly on a windy day, I just couldn't get the speaker loud enough so everyone could comfortably hear what I was trying to demo. Cranking the GPX's Target Volume beyond 16 just makes the unit very "on edge" and touchy. So I quickly started using external boosters on training days. A Dual speaker with a B&Z works very nicely in this scenario. After the usual "gear interrogation" by some of the trainees, I often had trouble trying to convince them that I generally prefer headphones