Yamaha makes receivers with pre-outs too. I own one, but it's discontinued since I've had it for a while.
I wish mine did...I love what it can do but I'd really like to add an amp and maybe even a few more speakers outside...
You know anyone who's good with electronics? They can open up the unit and pigtail the preamp signals off the mainboard before they reach the amplifier circuitry. It's not that hard (for an electronics enthusiast). Older receivers will use discrete transistors. Newer receivers will use purpose-built multi-channel integrated circuits. Look up the pinouts of the part number online and find the input pins. There's probably plenty of space to chassis mount some RCA jacks on the back of the receiver. Just a thought.