The MXL 990 is one of many Chinese-manufactured condenser mics that suffer from two common problems - it is simultaneously ill-defined in the critical midrange and excessively harsh in the treble range. While the circuit used in the mic is quite clever and fairly well implemented, the sonic deficiencies lie within the condenser capsule and headbasket acoustics.
By and large, Chinese-manufactured mics use condenser capsules copied from classic Neumann designs. Unfortunately, these inexact copies fall short of delivering the magnificent midrange accuracy and smooth, sibilance-free top end Neumann capsules are known for. Most of the copied capsules deliver a sound that is often described as harsh, spitty, "eshy", sibilant or screechy. Poor headbasket acoustics compound the problems created by these capsules.
The MXL 990, like most microphones of its class, uses a multi-layer grille mesh headbasket. These multiple layers of mesh do protect the capsule from spittle and wind blasts - but at a cost. Multiple layer headbaskets have a very small percentage of open area in the grille. This means a great deal of sound is reflected back and forth between the capsule and headbasket. Cup your hands around your mouth or speak into a paper tube to get an exaggerated example of the standing wave artifacts caused by a restrictive headbasket.
What to do?
One of the great benefits of the MXL 990 and other similarly priced mics is low cost. With a very modest investment in the original microphone, it becomes an excellent platform for a microphone modification. The capsule can be upgraded and headbasket opened up.
With a suite of microphone modifications this mic, (as well as many other Chinese manufactured mics) can really become a solid performer that sounds much more like a professional tool.