Integrated Tube Amps with Subwoofer out?

DOH!!! I'm so sorry I misread this as an ask for a Tube PRE.

My bad.  The Luxman LX-380 will fit the bill, and let you put a cap in to use as a high pass filter.

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.