The Sony STR-DA1ES does not appear to have a "5-channel stereo" mode, in which the front L/R amplifier channels and rear surround L/R amplifier channels would both receive an identical stereo signal (and the center amplifier channel would receive a mixed mono signal), as do some receivers like Rotel, Denon and others. The Sony STR-DA1ES only appears to have 5-channel DSP modes which would not give the same stereo L/R signal to the surround channels, but instead send them a reduced level delayed ambience signal.
If you were to connect each pair of 8 ohm speakers in series for a 16 ohm nominal load to your present receiver, it would probably produce limited power (although it would be a safe load). If you connect each pair of 8 ohm speakers in parallel for a 4 ohm (or less) nominal load, the receiver may not be capable of driving that lower impedance load at decent volume levels without overheating.
The Sony STR-DA1ES (and maybe your present receiver?) has an impedance selector switch that should be set to the 4 ohm position if you are using less than an 8 ohm speaker load as would be the case with 2 - 8 ohm speakers in parallel.
Many solid state amplifiers (without transformer coupled outputs) will output 40% or more power at 4 ohms than at 8 ohms, however on amplifiers like the Sony STR-DA1ES that have an impedance switch to prevent overheating and unstable operation with less than 8 ohm loads, they actually have a lower rated output (80W/ch) at 4 ohms than at 8 ohms (100W/ch), due to the switched in current-limiting circuit.
Steve
