You are thinking exactly right. I too could find no information on this problem. I have been using my Tact 2.0S with subs for a couple of years using the second line output (with a bal to unbal adapter) from the Tact. I have also tried the speaker level, and it works too. (Do not try speaker level with a Tact digital amplifier without further instructions.)
I looked at the in-room graph of my main speakers (full range) and it showed a peak at 80HZ valley followed by a lumpy mess going down. I used my subs to fill in the hole crossing the subs at 55HZ. The Tact will equalize the whole mess as if it is one speaker, thereby rolling off the mains.
I suggest that you cut the subs off sharply at their -3db points. If you use -6db you eat up too much amplifier power for most setups. I cut at 30HZ.
Good luck.
I looked at the in-room graph of my main speakers (full range) and it showed a peak at 80HZ valley followed by a lumpy mess going down. I used my subs to fill in the hole crossing the subs at 55HZ. The Tact will equalize the whole mess as if it is one speaker, thereby rolling off the mains.
I suggest that you cut the subs off sharply at their -3db points. If you use -6db you eat up too much amplifier power for most setups. I cut at 30HZ.
Good luck.