It is impossible to damage a speaker by "underpowering" it. Likewise, reducing the power supplied to a speaker will not cause more distortion. Someone has been giving you bad information. I recommended replacing his OEM (Original Equipment Manufacturer) speaker with one that would handle more power because it sounds to me like the OEM speaker has been damaged by high volume levels. Good engineering practice is to spec a speaker that can handle twice as much power as the amp's rating. It looks, from the radio's specs, that Uniden did just that. But I think the speaker they are using isn't really as hefty as the specs show.
Allow me to clarify what I was trying to say, in other words, I believe the amp is underpowered, causing the signal to be more distorted, because I find myself turning the volume to 100% to hear it sometimes and I believe this may be introducing distortion into the output to the speaker. If the amp was more powerful I believe I wouldn't have to keep the volume at 100% and therefore I would not be destroying that 5 cent speaker they put in there.