This valley or dip in the midbass area is very common in the Beyma woofers, midranges and fullranges, some have minor downhills, this 10AGN have a wide low area really.
Maybe stuffing the inside box with something suited, could rise some bass freq. This drive is old in the Beyma catalog and they do not fix it, nor release a new better sucessor driver, nor cast frame it have due the low price. The interest is to sell...