20 bit players vs. 24 bit players

Limited advantage unless some digital processing is done - in which case the extra bits help.

Main advantage is in upsampling - this helps improve filtering and can reduce jitter if done well.

"if most redbook cd's are only 16 bit??" - what do you mean by "most". All redbook CDs are 16 bit. Sony-Philips never created any other definition. Even HDCD is recorded in 16 bit format where bit 16 (least significant) switches dynamic range of music encoded in 15 remaining bits.

Ok' ALL Redbook cd's

Yeh but what's the SONIC Advantage??

Extra bits come from digital filtering - pretty much useless information. Sonic benefit is, as Shardone explained already, improved filtering with possibility of reduced jitter. Improved more accurate filtering in digital filters gives smoother more coherent sound while reduced jitter lowers noise floor.

I would not put too much attention to number of bits or specifications in general.