Dear Greg: I'm sorry to insist, the magic to integrate a pair of self powered subwoofers in true stereo way belongs to the crossover frequency selected: 38Hz/40Hz is to low crossover frequency, is a wrong one.
The first target ( and where the magic is ) when we integrate sub's is to liberate the main speakers ( Virgo ) of frequencies below 80Hz, these frequencies will take it for the sub's down to the very low bass octave. In this manner you will have the low bass ( foundation of the music ) that your main speakers can't handle/give and at the same time your main speakers ( that now are liberated of the frequencies below 80Hz ) will shine with a great mid bass/mid range/high frequency clarity, transparency, natural tonal balance that you can't have it at this moment and that a full range speaker can't beat.
Greg, the crossover frequency must be near 80Hz: in the low pass and high pass crossover filters. What you are doing today does not help to improve the quality sound reproduction in your system and that's why you posted what you are experienced in your system: " Works well with good recordings... not so well with others... ".
The problem is that your REL's does not have a high pass filter, then you need an external crossover to set-up in the right way.
Like I alredy told you: a main speaker/sub wired in the right way ( with same quality speaker level build ) beats almost any non self powered full range speaker. No question about.
Regards and enjoy the music.
Raul.