I agree witH Werd that having a killer source is of utmost importance.
But I disagree on synergy. Synergy trumps most things when it comes to audio. Synergy won't mask a bad source, but down the line, synergy can play a great role in getting the system to sound right.
The problem is somethimes synergy is elusive, and sometimes happens when you least expect it.
I am not so sure of the single brand theory. Lets say they make a great amp, that does not neccesarily mean they make a great preamp or source, they may be good at amps! Preamps are another animal.
Of course they want to sell the complete lineup so they can sell you stuff. Proac makes very nice sounding speakers, but I wouldn't expect agreat preamp or cd player from them.
Porsche makes great cars, but I would not buy a Porsche dishwasher just because it said Porsche on it...
Now a Ferrari dishwasher.....that's got to be a chic magnet