I know of no other practical passive system other than the Hafler. The rear
speakers are wired "ground-to-ground" with the "hot" sides going to the "hot"
amplifier outputs.

Or to put it another way, the rear speakers are wired in parallel with the front
speakers, then the ground wires are lifted from the amp and tied together.
Electrically, both methods are exactly the same thing.

When I was in college 35 years ago, I used this system with my KLH Model 11 FM
portable, and it worked fine.

Try searching eBay for used equipment. A Sansui QS-500 decoder/amplifier would
be a good choice.