You can isolate your the preamp in the integrated and send the signal to another power amp - or powered sub. Basically, you're using your intgrated as your preamp in this configuration.
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The pre out on most integrated amps allows you to send a signal to a different amp than the one built into the integrated amp. This allows some folks to upgrade the quality of their amp but retain the preamp portion of the integrated. However this is not often the same as having a second output to drive a second amp with the first amp still ingaged. Integrateds differ so it would be best if you ID'd your integrated so someone with specific knowledge of it could comment, or you could review your manual. If you don't have one you might find one on the manufacturer's web site.
There are different kinds of pre outs:
One you use with jumper to the poweramp input(of the integrated amp) disconnects the poweramp section.
and another kind that will not disconnect the poweramp section and could be used together with different matching poweramp to biamp your speakers(again if such have two sets of binding posts each).