I think the 308's are good enough at moderate levels, but distortion seems (don't have much to compare to) a bit much too soon at elevated SPL.
A sweep tone doesn't care whether you have subs or not (offloading) so I don't see much help for the distortion levels in the midrange by adding subs and reducing the low range of the woofer.
You only get so much perfection in that price range (I know, price is a poor barometer of quality).
Roughly 75/85/95dB sweeps:
THD, dB relative to fundamental
View attachment 11319
THD, as %, Normalized to fundamental
View attachment 11320
The harmonics are easily heard on pure tone at the same levels.
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At moderate levels, they're fine, I use them all the time for TV, movies, and casual listening.
They're running right now (TV), hadn't even thought about it till I looked.