Tubes and Subwoofers

Sure do! I used to connect the subs to a second pre-out on the tube preamp but now I take the signal directly from the amps.

Possibly helpful: https://youtu.be/Wo5TwacV7-c?si=jf3xRhWFgk_WD1BX

[AI summary: This video provides a comprehensive guide on connecting a subwoofer to a stereo system. It covers various connection methods, including speaker-level and line-level inputs, and explains how to adjust settings like gain, crossover, and phase for optimal sound quality. The video also discusses the benefits of using multiple subwoofers and offers tips on placement for the best listening experience.]

I used the outputs of my tube amps to drive the high level input of my active subs for quite a while without issue at all.  The sub's amp is completely separate.

You should have no problem proceeding with your plan. I have small pair of REL's connected to a small SET with great results.

I also use a REL sub (T/7x) to supplement stand mount speakers driven by a tube amp.  Sub speaker level inputs are high impedance and won’t load a tube amp output.

Rel's prefered connection is via the hi level, speaker connection, for example. This is the way I have mine hooked up. Workes very well and no issue if its a tube amp. you also do not need to use the same taps as the speakers are on if thats a concern.