It seems that loudspeakers are either built with drivers in parallel or series. Is there any advantage or sonic differences in either case or is the crossover all important? Thanks 
Firstly, IMO the crossover is
extremely important.
Secondly, from hearing friends' experiences with various Magnepan speakers I can vouch that a 2-way Maggie sounds different when it has the same crossover point but has a series XO instead of the stock parallel XO.
Parallel sounded a bit more "detailed" but the series XO delivered a much more holistic and engaging sound. Of course, Magnepans - and I would think many other speakers - sound better when the (typically cheap) XO components are replaced with the same values but higher quality components. (The Maggie Forum has post after post describing the improvement in sound after doing this ... possibly, Maggies are "made cheaper" than other brands or possibly Maggie owners are more into DIY?
)
Regards,
Andy
