Many receivers, including the 1070 have pre-outs to use an external power amp. I was looking through the online user manual and noticed that there doesn't appear to be a config option to turn off the internal power amps when using the pre-out. I guess running the internal amp without a load won't cause any harm, or does the circuit detect the no-load condition and shut down the amp?

If it's like the 1050 was (and like many other receivers), the internal amps run without a load. Running a solid state amp with no load is not going to harm it.

That's what I thought. It also probably means you can bi-amp your speakers by using the internal amp to drive tweeters (through a speaker level high pass filter) and the woofers through external amps driven from the pre-outs through an active low pass filter.
You'd have to set the level on the external amp so the speakers were balanced though.

That is correct.