I don't know about the amp aging question. As for the bridged and balanced, neither apply to the 770.

Bridged -- Bridgeable amps allow two channels to be combined to act as a single channel for twice the power. So a two-channel 100WPC amp that is bridgeable can also be used as a single-channel 200W amp. None of the 770's channels are bridgeable.

Balanced -- Balanced connections are a different approach to component interconnects. Here's a definition: "Balanced audio signals utilize two signal-carrying wires and a shield, as opposed to the standard unbalanced configuration that employs the shield as one of the signal carriers. It is often used in professional situations and high-end equipment because it is much less likely to pick up external electronic noises, such as hum, static, or radio interference." The "electrical midpoint is grounded" in a balanced connection. Higher-end amps often offer balanced inputs as a way to reduce noise in the signal. It also requires that the pre/pro have balanced outputs -- very few pre/pros offer that (the Anthem AVM-2 and AVM-20 include balanced pre-amp outputs, as does the Classe SSP-75 and the Balanced version of the Lexicon MC-12, the MC-12B).

As for what the five other channels are doing when the 770 is handling stereo only, it will help your electrical bill by letting them be idle. It should also allow more reserve for those two channels when they're being pushed really hard.

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Gonk