One easy improvement to inexpensive MXL mics is to replace the ceramic capacitors in the signal path.

Original ceramic .001uF (marked 102) on the left, .001uF/100V styrene film replacement on the right. That’s the one to fix if you do nothing else.

The other ceramics that carry audio are the .22uF (224) ones at C3 and C4. There are options for them. First off, they are a bit small and limit the low frequency response of the mic. Larger values will extend the bass response. On the other hand, for close vocal or other work, you may not want extra bass, so .22uF may be fine for you. I intend to use these at a distance of a meter or more most of the time, so I increased the value to 1uF (105). In the 990 there is enough room to fit larger film caps. The 991 doesn’t have room for big caps, so I used 1uF / 50V monolithic ceramics. The larger value means there won’t be as much signal across the cap; that is, both ends of the cap will go up and down together at audio frequencies, and any distortion will be correspondingly reduced. Here is what the 991 looks like:

Original on top, modified on the bottom. Of course the leads need to be cut - I left them long in the photo to identify the changed parts.

Here’s a 990 with a mylar film cap at the input and C3 & C4 replaced.

There are other improvements possible. sells a super upgrade kit which includes lower noise transistors, better resistors, and many other selected high grade parts. Changing these three capacitors gets by far the most bang for your buck and time invested.