The stock electrolytic capacitors are junk by now. Replace them with Dayton Audio or Mundorf polypropylene caps (values: ~6.8uF, 12uF, 47uF – check your specific board). This single upgrade transforms the speaker—revealing treble detail that was previously muted.
The AT-40 employs a multi-element, air-core inductor and Mylar capacitor network for the mid and high frequencies. cerwin vega at40 specs
Cerwin Vega has a reputation for "cupped hands" midrange. The AT40’s 4.5" paper midrange does a decent job eliminating this. It handles vocals from 600 Hz to 4.5 kHz. While not as transparent as a studio monitor (like KEF or Dynaudio), it is vastly superior to the D-series muddiness. Expect forward, energetic vocals, not delicate ones. The stock electrolytic capacitors are junk by now