Integrated Tube Amps with Subwoofer out?

Primaluna has sub out

self powered sub?

where’s the crossover/filter?

IF you send everything except low bass to the amp, after sending lows to the self powered sub, then the amp no longer has to make low bass, and neither do your main speakers.

especially important using tube amps, reduces needed power, size, heat, offers optional placement perhaps.

and, choice of mains is widened

I tried unsuccessfully to integrate a sub using the line outputs of my integrated tube amp when I had it. Get 2 subs and go directly from amps to powered subs. Much easier and sounds much better, bigger soundstage, clearer, tighter bass and seamless integration. You do not need a sub out as others have said previously. 

Rogue Audio Cronus Magnum integrated amps have a variable output that can be used for subwoofers. That's what I use and it's fantastic!

Black Ice Audio (formerly Jolida) has a new line of integrated all-tube amps that all feature (mono) Sub outputs.