I really like the idea of the W371, and I plan to order a couple someday to go under my 8351s. I don't consider the W371 as a "stand subwoofer", though. I suppose you could call it that, but "stand subwoofer" is somewhat of an oxymoron in my eyes

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A true subwoofer(in my view), should only be playing sub 100Hz content, so that way it can be moved around the room independently to generate the most accurate response, without the fear of it being localized. The W371 performs well much higher in the frequency range than typical subwoofers, but to use that capability, you have to co-locate it with the front speakers.
Also the W371A only extends (-6) to 23Hz, and I generally look for (-3)16-18Hz extension. Very little music has sub 20Hz content, but it does exist, and it's an awesome feeling to listen to at high volumes.
I view the W371 essentially as a way to turn your Genelec stand mount speakers into Genelec tower speakers. Instead of selling a Revel Gem2 and Revel Salon2 in the same speaker line, Genelec just sells a Gem2, with the option of converting it into a Salon2 as a separate charge.
Whenever I do purchase the W371s, I'll treat the 8351 + W371 as a single speaker, and still cross it over to real subwoofers below 80Hz or so.