Remember that a crossover is a sloping filter. Most AVRs utilize only a 2nd-order filter (-12dB/octave). This means that the sub and mains have to blend at and around this point. Most people have never heard of let alone performed the SDT (Sub-Distance Tweak) to align sub(s) w/mains at the crossover frequency. Phase issues between the subs and mains could affect the sound.Even when I had just one sub, a Hsu VTF-3 MK3 Turbo, I was using 80Hz as my crossover frequency. This leads me to suspect that, although my non subwoofer speakers are doing the same job as before, the improvement I am hearing may be the result of my new subs being better able to produce lower frequencies with lower distortion than my old Hsu could handle.
@gwsat, have you performed the SDT since getting the FV18s? Did you ever on the HSU? I think others are also correct that your FV18s are loafing compared to the HSU.
Also, while I can't find the chart I believe the FV18 and FV25 start naturally rolling off at 80Hz which I've found to be a positive thing in regards to clean sound.
Some AVRs have an additional 2nd-order LPF for LFE which defaults to 120Hz. Matching the speaker crossover point cascades the filters resulting in a 4th-order filter and results in a cleaner sound IME. This was recently discussed in a thread titled 80Hz LPF for LFE I believe. Using the line-in on a Rythmik sub [by design] achieves the same thing as the 4th-order filter is preferred by most music listeners. 4th order low-pass on sub, 2nd-order high pass on mains.
Oh… also placebo effect @gwsat