Why unfortunately?
I have a friend who owns Sasha's with an all ARC system and they sound great. The 2-4 kHz dip is intentional and is part of their voicing. Dave Wilson wants them to sound this way
See the spatial average (A64) in Martin Colloms room taken from the Hifi-Critic review - it will describe what you listen in a real room.
Even rivals such as Magico have a similar voicing in some speakers.
IMHO, if you do not like them, you should get some other speakers. Anyway you should compensate for the bass deficiencies - this will not change the character of the Sasha and will change your perception of the whole speaker.
Just one different opinion ...