Elpelotero said:
I really don't think you'd need a pedal to play metal on a Rectifier. As mentioned, a boost pedal would be nice for solos. Otherwise, a distortion pedal would be an injustice to the amp. I've seen people do it and I'm left shaking my head.
The distinction between a distortion pedal and an OD pedal is important here.
A distortion pedal, no, definitely not, that would be silly. Like buying a Bugatti and then replacing its engine with a cheap Korean import.
But as for OD pedals...that really depends on which generation and subgenre of metal your ideal metal sound is. Like I've said before, I know more than just one or two metal players who consider the Recto distortion basically unusable for tight metal riffing (80s metal, thrash metal, etc.) without an OD pedal. In this case, you don't use an OD to increase the amount of gain (let alone replace the amp distortion with pedal distortion) but simply to tighten up its gain structure and response by cutting the excess lows.
The typical OD pedal settings in this application would be Drive at zero, Tone near noon or cranked (depending on the pedal and the desired sound), and Volume set to taste. That does change the feel of your amp quite a bit but does not ruin its unique character.