Invisibility and blending are a function of crossover point, slope, type of crossover (level matching & damping controls), number of subwoofers and (last but not least) matching tonality of subwoofer and woofer drivers. I have paid attention to all these and I get perfect blending and invisibility between 4" woofers and 10" stereo subwoofers using a modded Marchand electronic x-over.
