In current 5.1 and 7.1 systems, the surrounds and rears can have a broad spectrum of audio information. The old ProLogic systems might have been more limited in range, but not anymore. Of course in a true x.1 system, the .1 will handle the lowest frequencies for all the channels, but the mains and surrounds will still be expected to go below 500 Hz.

So, your E80s will work as surrounds, even though they may not be as space efficient or easy to place as a bookshelf or wall mount.