If you have speakers with built in subs (and amp), I would think that you'd want to send them a full range signal. But will doing that tax your external amp more? I know that your external wouldn't actually be driving the sub, but does the amp know that? It's still sending that signal right? So isn't it doing more work than if it was crossed at a higher frequency? Do these type of speakers normally have much higher resistance at the low frequencies to keep the external amp from being over worked?