"I was told that Y cable one to two is better than one to one line cable for better sound output quality. also the cable quality is important too."
What does that mean?
If your subwoofer has two line level inputs, driving both inputs (L+R) will add six dB of gain to the overall level. Nothing else will happen as one side of the inputs, usually the right channel input, is a mono summed signal anyway. If the receiver/pre amp/processor only has one LFE or subwoofer output, the signal has been made into mono at the source. Driving both subwoofer amplifier's inputs will only add a small amount of gain and nothing else.
Therefore, unless you have the level controls on the sub fully advanced and still lack sufficient volume to properly match sub to satellites, you needn't do anything.
The cable quality is as important as you wish to make it. A simple, well constructed cable of good quality should be sufficient. Should you hear any advantages by spending more on a cable, you are free to spend your money as you choose. Cables are a quickly found point of diminishing returns.
Do not remove the ground lug from a three conductor subwoofer amplifier's AC cable. Doing so will void your warranty. If you want to lift the ground connection, you can buy a ground lift at any hardware store for less than a dollar. However, be aware you are sacrificing a potential safety feature by dismissing the dedicate ground. If you live in an area with severe storms, I would strongly advise you to seek better counsel than what you've been given so far. Ask whoever gave you this advice why doing away with the ground would improve sound quality and do not accept a "just because" answer. This sounds like a dumb idea promoted by massive stupidity about a specific problem. If you do not hear the results of a ground loop, there is no benefit to removing the grounding lug on any equipment.
"I was told that Y cable one to two is better than one to one line cable for better sound output quality. also the cable quality is important too."
Sorry what I meant was sub cable has two kinds of cable I can find at market: one single ( one to one), the other is one to Two ( called Y cable?). My Marantz 7500 has one RAC output, is this call LFE? Comparing those two cable which one is better?
Regarding the ground, as sub got sound when nothing turned on, someone suggested that can get rid of that sound by remove or insulate the middle one on the power plug.
"sub cable has two kinds of cable I can find at market: one single ( one to one), the other is one to Two ( called Y cable?). My Marantz 7500 has one RAC output, is this call LFE? Comparing those two cable which one is better?"
A "Y" cable has one plug on one end and two plugs on the other end. It is neither better nor worse than a single cable. You use a "Y" cable when you need to go from one output to two inputs. It is a matter of necessity and not quality. If your subwoofer has two inputs, you will gain six dB of additional voltage for the same position on the volume control by running the "Y' cable into both inputs.
That is all that will change!
There is no quality change that will occur by adding the additional input voltage. Got it? If you have sufficient volume from the sub with just one cable, there is no need to run a line into the other input. Period!
"Regarding the ground, as sub got sound when nothing turned on, someone suggested that can get rid of that sound by remove or insulate the middle one on the power plug."
I'm sorry, but you need to learn to speak well and type better. Is this how you present ideas to prospective clients? The sentences you've posted make no sense. If nothing is turned on, then there can be no sound unless something is haunted! You cannot "insulate the middle one" on the power plug. I told you, do not remove the ground lug on the subwoofer's AC cable. If you would like to try this again with a proper question that actually informs me of the problem you are having, I will try to answer once again. But I suspect the second answer will be very much like the first I have already provided. Go to your retailer and ask for assistance if you have grounding problems with your subwoofer. It is their job to resolve such situations if you have paid them decent money for a product. If they suggest removing the ground lug from the AC cable, you got what you paid for.
