The Sony B series is their mid grade stuff and not too bad. They have similar quality to the A series (Sony ES) but without the 5 year warranty. I find the Sonys a little complex in setup and use. A bit about the S-master amplification, all of the previous digital amlpifiers that sony has released, including the current ES pieces, have a much higher Total Harmonic Distortion. Check the specs on this one and compare to the Denons <.08% THD. Also there seems to be a greater chance of defects with the sonys but maybe that is just my perception. The Denon has multi-room/multi-source although you get mono in the second room. Let me know what you choose and how it works out. Also, where did you see the str-db790. A search on google only got one hit.
You can see the Db790 at Sony's austric site - www.sony.at.
I don't know about the DB790 THD... The DB780 has 0.09% but without S-Master amplification. The 790 is the replacement of the 780.
THD specs is measured at a specific voltage output, almost at maximum, so why even bother about so small differences. I have only listened to digital amplification in a TacT millenium & sony mini hifi system & the technique seems extremely promising for the future!
Dude stay away from Sony! Their receivers SUCK BIGTIME!!!!! Honestly go with the Yamaha RX-V540 or the Yamaha RX-V640, they are better for music and movies!! Or you can go to the Denon AVR-1804 as well!
There is a up to 500 Eur receiver test in september issue of German Video (www.video-magazin.de, to read it you have to buy a paper edition I'm afraid) where STR-DB790 is a clear winner also in sound test (Yamaha's 540 got a couple points less for sound).