Sorry, little bored during lunch.
The OP should be fine given a known 20 amp circuit.
I just quickly gathered (and guessed) on some data for his equipment:
- Pulled from a 60 inch Sharp LED 3D LCD: 220 watts
- His AVR is rated for: 1405 watts max and 120 watts at idle (from the manual yet sounds extremely inflated for the amps: 85/channel)
- AT&T receiver: couldn't find data but probably no more than 120 watts
- Modem - basically negligible
- PS3: couldn't find data thus maybe 100 watts
- Mono Amp for sub? guessing 250-500 watts max
- Fish tank equipment? this could surprise ya - possibly 250-300 watts
So even if you went Max numbers above he "may be" looking at 2600ish watts. Thing is, under even heavy use his AVR and Sub amp would not be pulling max draw so he is fine. This however does not include his planned additional amp. But as Commsysman pointed out, utilizing his AVR preouts the external amp won't change much given he is decreasing the load from the AVR etc. My bet would be even on a "loud" day he would be "very" hard pressed to pull 6 or 8 amps / ~1000 watts total.
I personally however would try to remove the fish tank equip from the circuit if able. A previous college roommates "tank" always provided a nice hummmm to/in the stereo...