Q: I saw a video demonstrating the limited-edition Fender Hot Rod Deluxe guitar amp on another site, and the guy said that the preamp out connection was to feed an external power amp, then you should bring the power amp’s output back into the Hot Rod Deluxe using the power amp input connections to feed the internal speaker. Is that correct?
A: Absolutely not! NEVER connect the speaker outputs of any type of amp to anything but a speaker or a suitable power attenuator/dummy load or damage will result. (Always check the manufacturer’s recommendations before connecting anything other than a suitable speaker to a power amp’s output.)
On the Fender Hot Rod Deluxe (and virtually every other similarly equipped guitar/bass amp), the preamp out/power amp in is designed to function either as an effects loop or as a way to daisy chain amps together.
The preamp output connection is intended to feed effects processors and recording gear. You could also use it to feed the power amp input on another amp.
The power amp input connection is intended to receive signal from an effects processor or the preamp output from another amplifier. This input feeds the internal power amp of the Hot Rod Deluxe, which in turn powers the internal speaker and external speaker jack.
The Hot Rod Deluxe’s owner’s manual clearly describes how to use the amp’s Preamp Output and Power Amp Input connections.