Ideally, a converter won't have any "sound". It will just transparently pass through the signal. From the spec sheet you can check things like
- dynamic range/signal to noise ratio
- the amount of hiss that the converter will add to your recordings. 24-bit converters aren't inherently better than 16-bit. They need to actually have lower noise or the extra bits provide no information. The dynamic range spec tells you more than the bit depth.
- equivalent input noise
- the amount of hiss the mic pre will add
- total harmonic distortion
- distortion creates extra frequencies that weren't present in the original
- frequency response