Here is a potential fix that I can’t recommend enough:

1. Get Yaxi stPad2 ear pads
2. Unscrew the 4 screws holding the driver plate to the wooden cups. They are Torx T6 screws.
3. Gently pull the driver plate away from the wooden cups. You'll see that there is a piece of open cell foam that is sitting there in the cup. There are two gaps of exposed wood on the left and right. Take a single cotton ball and pull it apart until it is really wispy. Take a SMALL amount of cotton and fill in the left and right gaps
4. Above and below the foam there are two more gaps, but these need to remain clear as you’ll notice there are vents on the driver plate for these areas. That said, take a very small line of cotton and attach it where the foam meets those gaps. The intent here is to have a small filter between where the sound is entering those gaps from underneath the foam. Make sure you don’t block the driver vents in any way as that will muffle the overall sound.
5. Seal the headphones back up and put the Yaxi pads on. Make sure the Alcantara portion of the pads is facing FORWARDS. If you have nice taste and bought the Red and Blue pads, the Blue pad is right, the Red is left, and the pad seams are facing down.
6. Listen to the headphones. If they are too bassy, add a little more cotton into the cup without blocking the driver or vents. If they aren’t bassy enough, remove some cotton and/or break apart your cotton further.
7. Here's a quick picture of what mine looks like:

The end result for me is that the headphones are much cleaner and more detailed than before. The bass is tight, mostly clean, textured, and still emphasized as the headphones were designed to be. Further, the mids and highs sound better now that the bass is confined to it’s own area.

Source:https://www.head-fi.org/threads/meze-99-classics-discussion-impressions-thread.794593/page-310#post-14684508

My pad search is finally complete! After trying 10 different pads I have found, for me, that the Yaxi stPad2 ear pads are the ideal solution for the 99C’s. It turns out there are two causes for the bass bloat in these headphones. Lack of damping in the ear cups & the design of the most recent stock pads. I think I’ve figured out how to fix both of those issues now. This post will be a quick breakdown of the Yaxi vs Stock pads, and the next post will be some additional changes that further refine (for me, fix) the headphones.

Comparing the Yaxi pads to the stock (3rd version) pads, the Yaxi pads:

• Push the sound stage out a bit, but nowhere near the effect of BW velours or typical larger pads. The change is subtle enough that it just sounds like the sound stage is slightly larger. This isn’t offensive, and if anything it actually provides for better instrument separation
• Reduce lingering bass. Bass quantity remains the same to my ears, but the lingering bass issue is greatly reduced (but not eliminated). I think this is probably due to the alcantara section of the pads allowing for the bass to escape a little.
• Push the treble up a bit. This again is similar to how velours sound, but to a much smaller degree. This isn’t a desired change for me, but the change is subtle enough to not be a big deal.
• Have an airier overall sound, a minor effect. The headphones sound more “Open”, which is good in my opinion. This also seems to reduce the feeling of sound pressure which is another positive change for me.
• Comfort is about the same. My ears touch the drivers for both pads, but a little more so for the Yaxi’s.

Source:https://www.head-fi.org/threads/meze-99-classics-discussion-impressions-thread.794593/page-310#post-14684496