I agree with @audiotroy's analysis. In the bass region, the speakers are "seeing" a huge airspace.
Also, eyeballing Stereophile's measurements from June 1999, the Amati is not a bass monster. Its efficiency is on the high side, and (for a given enclosure size) there's a trade-off relationship between efficiency and bass extension, so "not being a bass monster" is no surprise here.
The slope of the Amati's low-end rolloff is fairly gradual, which to me implies that it might have been designed for fairly small rooms and/or placement close to the wall so that it would benefit from the resulting complementary rise in the bass region due to boundary reinforcement.
@torke, I suggest you enjoy how they sound with the subs and not worry about the fact that they "need" subs. Imo that's just the nature of this particular beast in what is effectively a very large space at low frequencies.
Duke
speaker manufacturer