All tube amp should have this option, as it’s a few dollars in parts and easy to do.
I used to put them on my tube amps, on or off. And yes no/less feedback = more amplifier gain.
When the feedback was switched on, I also had a dial you could turn to vary the amount of feedback from almost zero to 35db of feedback. This also lets you tune the amps damping factor to the speaker, depending if the speaker was an under or over damped design.
Cheers George