Hi, i recently got a used micro cube for $40. It works but sounded like crap, breaking up and complete distortion anywhere near 1/2 volume. I pulled it apart and found a tear in the speaker, searched some guitar forums and found that people are swapping the speaker for a 6" with some modification to the cab. The original speaker is junk. Paper surround that has basically no movement. Im into vintage audio and have refoamed and recapped some speakers so i really think this is a great opportunity to make a noticeable upgrade for minimal cost and effort.

I didnt go the 6" route. I got a Boss BRS52, 5.25", 60watt, 4ohm replacement speaker from Amazon for $10. Nice foam surround for plenty of movement of the cone. I used a side cutting bit on a Dremel to open up the whole for fitment. 

Plug and play! Sounds awesome. 

There are utube vids about the swap.