Uniden intentionally limits the level of audio available to the earphone jack so people don't damage their hearing. The earphone jack is a higher impedance than what you typically find in external speakers which are typically 4-8 ohms. As I recall the impedance of the internal speaker on the SDS100 is 32 ohms. A lot of earphone are as high a 600 ohms or more.
High quality communications gear, whether it's a receiver, speaker, or headphones does not necessarily mean better sound, but rather better intelligibility. Typically it is very limited in fidelity (audio frequency range) and will sound flat and sometimes even a little harsh. Audio frequencies are clipped above and below the response we're used to. This is make the voice as clear as possible, and not sounding like Barry White or Mariah Carey.
If you still want a fuller sound from a hand held scanner I think a Bluetooth transmitter plugged into the headphone jack, transmitting to a nice Bluetooth amplified speaker is the best approach. That way you don't have to worry about overloading/overheating the internal amplificer in the scanner.