It's always good to have more options and tools at our disposal. Depending on the preamp and power amp (some but far from all of us have McI's), boosting the instrument signal into the preamp may sound better or worse to us (or be more or less easy or practical) than boosting the line level out of the preamp into the power amp or modding the power amp.
Disclaimers: I'm happy to share whatever relevant gear knowledge or experience I have. Genuflection/payment/proof of worthiness neither sought nor accepted. I don't design, build, market, sell or endorse for pay any new music product or service. I occasionally sell personal used gear. I am an unpaid endorser of SoundScaper Productions and Godfrey Daniels Guitarworks.