The weight of surround receivers nowadays is mostly due to how large of transformer they put in for the main power supply and what kind of heatsink they use for the amp boards.
The heaviest I know, Denon AVR-X8500H, comes in at 51lbs because it has to pack in 15 separate amplifiers with somewhat thick heatsinks and a transformer large enough to supply. Second would be something like NAD 777, which uses a centralized heatsink with very thick metal fins to support the amplifier boards.
I just recently opened up a Yamaha RX-V2070 receiver that I used in a friend's application. It comes in at 37lbs, which is not light for today, but not in the heaviest area. The amp board transistors are mounted to a good thick heatsink base, but the metal fins are very thin (I assume to save on weight). While it may not be heavy, the heatsink does it's job and the receiver doesn't get very hot. It is probably biased very low (maybe 1/2 watt in Class AB mode).
The components in the Yamaha were actually pretty good, using upper grade Nichicon electrolytic capacitors and all through-hole components on amp boards and analog boards (no SMD for audio). I decided not to mod it at this time because it really wouldn't be worth the hassle to pull everything apart.
I don't know what's in the Onkyo and other receivers, but none of them sounded good to me - the receivers are usually bright and too "solid state" sounding. Out of all receivers, I respect Yamaha, Marantz and NAD. Both Marantz and NAD use good components and sound good, but their sound is too warm for my tastes. The Yamaha was the most natural sounding receiver (in my opinion) and had the most resolution and attack.
