As rare as hens teeth? Not quite, but close.

slider313 · July 28, 2013

Yep. The Bassman has a bigger, tighter and cleaner sound, with higher B+ voltage and bigger transformers. The Vibrolux is more like a big Deluxe. As a matter of fact, the Vibrolux and Deluxe run their plate voltages around 410v, even though one runs 6L6's and the other 6V6's. This gives the Vibrolux earlier breakup than its larger siblings. Something you may or may not want.

One reason your Supersonic may sound thinner on the Vibrolux setting is the switch may lower the B+ voltage, which in turn would really require you to re-bias the amp accordingly. Fender is known to send their amps out cold biased, which helps them avoid power tube warranty issues, but isn't exactly good for tone. Your amp may be set slightly cold to begin with and switching to the Vibrolux setting is making the tube run even cooler.

slider313 · July 30, 2013

Wow, one just turned up for sale on The Gear Page! It's real clean! You don't see these very often. This model was made for a little more than six months; from March of 1964 through September of 1964. In September of 1964 the newly designed Vibrolux Reverb was in the works and released in October to Fender dealers.

http://www.thegearpage.net/board/showthread.php?t=1297420&highlight=vibrolux