The E8 are much more "breathing" with a fuller sound, no noticeable difference in the highs department but the mids are much positively affected by the bass extension.

it is not certain at all that the E5 + sub connection will produce such harmony and cohesion.

my friend had some bassy soul/funk tracks and the E8' s low end is really there, tight and punchy.

for the D8, I heard them very briefly in a noisy
environement some time ago,

but the D5 didn't impress me, considering the big difference of price with the E5, so
i can only assume that the difference of price/quality ratio would stay more or less the same between the two 8 inch versions.