Integrated Tube Amps with Subwoofer out?

This is a non-issue. There’s two simple solutions. Easiest is a LOC or Line Out Converter. Next easiest is to modify your integrated. The mod costs about $2 total for 4 resistors and can be seen in its entirety here.
https://systems.audiogon.com/systems/8367 Look at #26 of 47. That’s a close-up of the 2 resistors.

All you are doing is adding one 10k resistor to drop the voltage, and another about 1k to provide input impedance to your sub. The complete circuit diagram with explanation is here. https://www.epanorama.net/circuits/speaker_to_line.html

If you buy a LOC that’s all it is, two RCA connected to 4 resistors just like the circuit above. The LOC is cheap and easy but looks tacky. The mod is a little bit more difficult but looks factory original. Same result either way. So if you buy a LOC don’t waste your money, just get the cheap one from Parts Express for around $10.

The Raven is of course totally the way to go, and will be better in every way. Their sub out is even switchable. Check it out.

The Parasound HINT has a subwoofer out.


Sorry - didn't see the "tube" qualifier.

You can use this for $40- Russound ADP−1.2 Speaker Level. I used this to run dual subs out of Leben amp.

Itis not necessary to have a preamp out for a Subwoofer,crutchfield sells a adapter 
you just put a awg 16/wire in the + - for each side of adapter ,and either a 
banana or spade into speaker terminal it is actual more accurate 
and crutchfield , or call SVS subwoofers they can tell you exactly what it is called for under $30 I used one with my Krell integrated worked great. 

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.