Avoid the speaker level setup if you can. Many subwoofers have speaker level ins & outs, so if you go that route, use the high pass output to your main speakers. That'll work better than what you've sketched.
Best solution is run your tube preamp's main outputs to the sub's inputs, then use the sub's high pass output (line level) to the main amplifier's input.
Technically speaking, here's why the line level setup is the best application:
By using the subwoofer's on-board crossover, you're limiting the low frequencies the main amplifier is receiving, since low frequencies would now be handled by the subwoofer, via the on-board crossover, typically adjustable by the user to best suit the equipment and the listening environment. Thus the main amplifier won't be working as hard. Speaker level ins / outs have the amplifier running unrestricted, and thus is not as efficient for many reasons.
HTH!