70Ω is not what I expected. Audiophile headphones are usually 4, 8 or 16Ω. Earbuds and light duty headsets that come with MP3 and IPods are usually like 35 to 70Ω. Aircraft headsets are like 500Ω.
If you parallel 8ea 70Ω headsets, you have about 9Ω, which is within the range the amp will like, so you don't need the resistors. Just set the master amp level to get sufficient loudness. If some of the listeners are hard of hearing (not uncommon amongst musicians and pilots, ask me how I know), you may want to get 8 stereo L-Pads (fancy ~ 10Ω pots) so that you can custom set the levels for the musician that haven't lost their hearing yet.