A bit more background...
The Sony VFET power amp designs was Sony's first attempt using high-current FETs as output devices. At the time, Sony and most of the other Japanese companies had begun to take on the higher end manufacturers like Mac, Phase Linear and Crown.
They did this by introducing new designs like the VFET stuff in a unique and high-priced separates package like the TAN series. Later some of the design features first seen in the high-end Sony line would begin to show up in the receiver line when they rolled the product line for the next generation. You can see much of this in the Sony units throughout the 70s, too.
The problem was that the VFETs were not that good and stable. To add to the confusion, most bench techs didn't have the knowledge or the equipment to fix them. This tarnished the top end Sony line a bit and, as you can see, they didn't continue this practice in the 80s.
The store I worked in sold 2 set of these preamp/amp sets but they were quite expensive - almost on the order of Mac equipment. Even today, given the choice, I would take the Mac over a Sony any day.
Hope that helps.
Cheers,
David