Sure, except for using speaker level connections. I assume your electronics forces this choice.
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Line level connections provide a convenient path to use bass management. REL subs were originally (maybe they still are) called "sub bass" speakers. They were designed to augment the sub bass of speakers that were close to full range; lacking in maybe the lowest octave. If you don't need to bass limit the main speakers, then either line level or speaker level should be equivalent. Speaker level connections gave them a larger market.
There seems to be plenty of differing opinions on whether main speakers should be bass limited or not and that appears to drive many opinions regarding line level vs speaker level sub connections.
Richard Vandersteen is the only speaker designer I know that bass limits his stand mount speaker. He also high passes his floor standing speakers, some of which are considered full range, when utilizing his subs. He seems to accept the physics of producing clean bass and designs accordingly.
The OPs B&W 805 has some distortion data published in its literature: http://www.bowers-wilkins.com/Downloads/Product/InfoSheet/ENG_FP29483_805-Diamond_info_sheet.pdf. You'll see that the distortion doubles, from 0.5% to 1.0%, when the frequency is decreased from 150 Hz to 100 Hz. The spec is for 90 dB at 1 meter. So it'll be worse at a typical listening distance. Recall that 1.0% is generally considered clipping for amplifiers. Why doesn't B&W state the distortion at 80 Hz or lower?
See distortion graphs for main speakers here: http://www.soundstageav.com/speakermeasurements.html
And subwoofers here: http://www.avtalk.co.uk/showthread.php?t=12281 and http://www.hometheatershack.com/forums/subwoofer-tests-archived/6015-index-subwoofer-tests-manufacturer-model.html
It's clear to me that the vast majority of main speakers would be cleaner being bass limited. Of course, clean sound may not be everyone's cup of tea.
so if i can only use speaker level to feed subs, would not the subs internal adjustable crossover be sufficient to low pass all bass frequencies below the main speakers lowest frequency?
for example, i have my current sub crossed over @ 30 hz, (its lowest x-over point, btw) while mains are picking up @ 28 hz on up. (published specs)
with aforementioned 805's i would probably crossover much higher for obvious reasons.
I, too, have the Rives test tone CD lying around somewhere. I don't recall the frequency resolution of it. Many test tone CDs use 1/3 octave frequency spacing and that's not a fine enough resolution for bass work. Dr. Floyd Toole suggests 1/10 octave as the minimum resolution.
The free download from Real Traps, I think, is every 1 Hz. If you can burn it to CD, it's probably worth the effort.