Trying to wrap my head around how this is worded...
"as a power amp" - no.
"between a modeller and passive guitar speaker" - no.
You could use the CaptorX in between your modeller with its cab sim turned off into a powered studio monitor - yes.
You could use the CaptorX after your modeller with the cab sim turned off into a powered FRFR speaker like the F12-X200 if it was independently powered (ie: similar to the studio monitor setup)
The CaptorX is designed to both "attenuate" and "direct-record" via XLR/TRS outs, or silent play with "headphone out". Your modeller - assuming its one of the several obvious one out there - is supposed to be doing this for you all the way out to the studio monitors if going with cab sims, or to a cabinet with cab sims turned off and a power amp feeding the signal out from the modeller.
Does this help?
Mo