Build quality will be extremely comparable for the two amps -- the cost difference is purely a matter of material and manufacturing required to generate the additional power for the 770. The concensus from those who have heard both amps is that sonically they are not much different unless the combination of speakers and material begins to demand a lot of power. See ndrayer's post in this thread for a first-hand account. There's also this recent thread (twin to the previous thread, but in this forum rather than the 770 forum) which has some good information. Likewise, this thread might be useful to you. Both the 7100 and 770 are excellent amps.

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