I Do not know of any of the Fox Pro units that need extra speakers. The ones that are built in are more than plenty except on a few special hunts for crows, geese, or on perhaps high wind days. In fact I think we get into a habit of playing our sounds too loud.
I have used the shockwave since it came out. The volume is ample for the calling I do in eastern Montana. I play max volume 95% of the time. The call does have enough volume for all but the windiest days. I recently bought an XWAVE, but have not used it yet, so I am not sure if it is louder or not.
I actually use 2 fox-pro callers. One no more than 40 yrds in front of me and hold the other one in my hand. If nothing comes in within 5 minutes or so, I turn the one in my hand on and make a slow sweep in 360 circle then turn it back off. That seems to bring in more from behind me.