20 bit players vs. 24 bit players

"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.

The difference between 16 and 20 bit quantization is an expansion of dynamic range, as well as increased low-level detail resolution. 20 bit resolution actually represents the state-of-the-art with regards to digital audio reproduction. Try some XRCD material to experience 20 bit resolution on your Redbook player. Some DACs have 24 bit capacity, but the last 4 bits aren't precise enough to encode music accurately. Some advertisers claim 24 bit resolution as a marketing ploy. The short version: There is no sonic advantage between 20 and 24 bit(yet).