Which is best...rca splitter or Y cable splitter?

Which splitter method will cause the least signal degradation? The solid monster type rca splitter or using a Y cable, to split a signal from a pre-amp?
Niether method is going to provide quality results. Which preamplifier are you using, and what is your intention? If you want two pre-outs to biamp, your probably best off getting a preamp that is engineered to support two sets of pre-outs.

A lot of people will probably disagree with me on this.

Another route if your are trying to biamp is to use two identical amps that can be daisy-chained. I know Linn does this, and I'm sure there are several others.
Electrocompaniet also does the daisy chain method.