Integrated Tube Amps with Subwoofer out?

Primaluna integrated amps have one. All tube. 
Decware has a line out intended for subs as an option. They add tube rectification which is awesome too. 

Use any integrated you want. Simple elegant solution. Give Jensen a call and they will set you up with any type connection that fits your situation. Under $200 I believe, very friendly service. 
https://www.cs1.net/products/jensen_transformers/SC-2NR.htm

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.