I've had very good luck with my REL sub and Sonus Faber Electa Amator I's, Signums, and Silverline SR17. I suspect that some problems come when people use low pass filters to feed their monitors less than the full range signal their crossovers were designed to anticipate.
I think it's important to first get the main speakers as dialed in as possible without use of the sub, and then add the sub in. After a while with the Signums, I had to go back to the beginning and get them positioned for the least bass, consistent with good imaging, and adding the bass from there.
Don't rely on frequency response numbers shown for the main speakers for a cutoff point, and try all the phase settings for the best match. It's easy to spend a week or two to get it all sorted out with a new combination.
I think it's important to first get the main speakers as dialed in as possible without use of the sub, and then add the sub in. After a while with the Signums, I had to go back to the beginning and get them positioned for the least bass, consistent with good imaging, and adding the bass from there.
Don't rely on frequency response numbers shown for the main speakers for a cutoff point, and try all the phase settings for the best match. It's easy to spend a week or two to get it all sorted out with a new combination.