My guitars require the 18db pad on Input 1 (LP's with JB's in the back), to avoid clipped waveforms as tested via Pro Tools. YMMV?
You want guitar level in, guitar level out to your amp, whatever your amp's FX Loop sends (typically line level but not always), and whatever your amp's FX Loops's return wants (typically line level but not always).
Good luck, works fine here as long as you properly adjust the above, and importantly, properly set Input 2's Noise Gate's threshold.
Tube gear is inherently noisy, so setting the Input 2 noise gate should minimize the guitar amp's preamp's noise when you're not playin