The specs for that amp are very interesting as it happens.
It's rated at 85W with a decent THD.
Stereo is rated as 100W with a higher, but still decent THD level.
It's rated at 140W with a rubbish, but probably acceptable THD level.
But if you read carefully, it states;
"reference power output for front, centre, surround and surround back speakers.
Depending on the sound field settings and the source, there may be no sound output".
What use is that?
All pretty meaningless and I'm digressing a little, sorry.
In this case, I'd look at the stereo rating (100W) as a reference.
If that's what it puts out in stereo you can expect 5.1 surround to be significantly lower and 7.1 to be lower still. The limiting factor is usually the power supply and if it's nearly flat out in stereo, it makes sense that 5.1 will be nearer 40W per channel in reality (although it's not quite that simple). This applies to most amps, so hopefully sort of answers the question, in that conversely, if you use two less channels, you will get a bit more power to each channel in use. All this assumes each channel is going flat out, but in reality that doesn't happen (just to confuse you a bit more
).
Very misleading, but Sony are not the only ones who do that sort of thing.
Onkyo, for example quote power via one channel running!!!
A much better indicator is to look at power
in
In this case it's 230W, which in all honesty isn't brilliant.
Even then, it's only an indicator though.
I'm not saying it's a poor amp, it's probably about right for the money.