I dont have front channel preamp outs on the receiver, so an auto switching power amp for the front L and R wouldn't work.
You can use a speaker to line level converter. It takes less than $1 (assuming you use 1% resistors) worth of resistors per side and if soldering isn't your thing you can buy ready-made units for the car audio market.
To answer those curious and for posterity, I went with a Niles SPK-1 that I picked up cheap on eBay. It automatically switches sources when I fire up my stereo amp. I just finally wired everything up today and so far so good.