My understanding is it's a simple padded input (6 db, perhaps?), though I'm no tech so perhaps others will chime in with more specific info. Many types of gear have available padded inputs, e.g., most PAs (for regular and condensor mics), acoustic guitar amps (passive and active PUPs), and of course, electric guitar amps.If you Google "twin reverb second input" and/or "amplifier padded input," I'll bet you'll find lots of discussions. What I'd do in your position is take 5 minutes to experiment with yours and listen. This will tell you more than anyone else's descriptions.
“[P]ersonally I disagree with the general philosophy of ‘genre equals gear’ at anything beyond a cosmetic level. I think the right idea is to find the right amp for you, and then play whatever kind of music you like. The “Jerry Thing” was just Jerry. It wouldn’t have mattered what he used and he used a lot of stuff.” Steve Kimock (The Gear Page 04/08/18)