How to control volume on an integrated and amp

Shortly I will be getting my 47 Labs Shigaraki integrated which will run a pair of horn loaded Zhorn speakers. Nevertheless, because of my experience with running almost full range speakers and the practical lack of tactile bass of SET+horn fullrange speakers, I am thinking about running the set up along with a DNA 225, controlled by a Paradigm X-30 crossover to a pair of freestanding passive subwoofers.
The problem resides in the fact that when I run the set up with a preamp, one can control the amp that run the top end, while another variable output might go to subwoofer amp. Thus, when the volume goes up and down on a preamp, it will increase the volume of the high amp and sub amp. This is out of question with the Shigaraki, which has only two outputs, a line output, fixed and the output to the speaker posts.
How will you run a set up that can control the volume across all frequencies, besides resorting to buying a powered subwoofer?
Get a speaker-to-line level adapter. Common from the auto audio guys.
Hey I'm looking to to the same with a integrated 300b tube amp. Get any good answers yet ?
line adapters are useless...since the inception of this post, I got one, did not work as stated, returned it. I sold the xover and gave up in the idea of running a passive pair of subs, with the xover and the extra amp. It is much easier to get a powered sub, with high level inputs, and run two pairs of cables from the speaker output binders of the integrated. Now, there is two sets of speaker cables, they run from the same output, one to a pair of low frequency limited sets of speakers and the other to the subs. Signals travel at the same rate, thus no timing issues. The sub that I got is very fast and powerful, so, I think I am getting everything I wanted and the motive behind me writing the post, without having to go crazy with multiple RCA wirings here and there. Now, the blended signal is most definetly seamless.

It's too bad that the line-level adapters you got did not work for your setup. What did they (not) do?

OTOH, I have built them from pairs of resistors and they work just fine.