If you are using the sub output to feed the 8-inchers, then you are only givng them signal below the sub xover (maybe 80hz), when their internal crossover is probably designed at more like 250 Hz. Your lower mids and upper bass are probably non-existent. That would certainly keep it from 'getting loud' ;-)
Feed both sets of binding posts a full-range signal, or just jumper them and use one good 150-200 wpc stereo amp, and you'll be fine.
Feed both sets of binding posts a full-range signal, or just jumper them and use one good 150-200 wpc stereo amp, and you'll be fine.