I have one of the balanced monitor outputs on my mastering console permanently connected to an Orchid Electronics Amp Interface (a passive transformer reamp box with ground lift, polarity reverse and attenuator. I love it, just as good as the Radial boxes but cheaper). This means by switching away from the main monitors and to the reamp, I can send any source (analogue or digital) to the reamp. What the reamp is connected to is just a guitar lead, which can of course be plugged into anything in the studio. It’s usually plugged into my Deluxe Reverb clone (tube guitar amp) for recording with a mic, but it can just as easily be plugged into the Eventide H9, a number of different guitar effects boxes, my Moog Grandmother (great for the filters, spring reverb and crazy AM or FM shenanigans), which are then connected back to the mastering console for recording.
It’s an extremely versatile setup with some truly astonishing results, but takes time to set things up. It’s nice to dedicate a day to just reamping and re-recording stuff. Coolest thing I did recently was send a lead vocal through the Moog and tube amp that was slightly overdriven and spring reverbed, and then mix that back in subtly with the regular vocal take. Lent it an extra special something!
But really the sky’s the limit with reamping. You can also just run things through your monitors and record them with a stereo pair of mics for real room sound, no reamp box needed then! AFAIK Martin Hannett (Joy Division producer) did a lot of this.