Integrated Tube Amps with Subwoofer out?

Any pre with a pair of outputs will work, as will a Y splitter.

Here’s the trick. You need to make sure the sub and amp input impedance are high. So long as that’s the case, you are good. 
If you want to put in a high pass filter in the amp, you can build one with a single cap.

you should ideally use speaker outs (not pre out) to drive a sub (or better yet a pair of subs)... rel and other firms do this, many (myself included) feel this really helps the sub(s) work and integrate better for hi fidelity stereo sound

Agreed, many subs these days have a “high level” input that is directly connected to the speaker terminals to deliver the signal. As mentioned, REL is one such manufacturer. This method opens your door for many other integrated amps. 

If you do go with speaker level output from the amp to the subwoofer the Raven Audio tube amps have an interesting feature, a pair of speaker outputs with a selectable high-pass crossover at 80Hz or 100Hz that means those bass frequencies are removed from the amplifier section and passed off to the subwoofer, effectively make the amp 2 to 3 times more powerful, according to Raven. My Blackhawk was built before that feature was added so I can’t testify to its effectiveness. When I had an HSU subwoofer the high-level outputs were what were strongly recommended by Dr. Hsu.