how can a line cord affect frequency response ?

The AC cable does not change the frequency response but rather allows the electronic component to perform more or less efficiently due to the restricted or unrestricted flow of electrons. Try a Radix Cable; the Quiddity will be your benchmark for efficient electron transfer and @$250 it won't break your piggy bank. You can find them at AudibleElegance.com or RadixCables.com for the same price. You will be pleasantly surprised.

". . . restricted
or unrestricted flow of electrons."

What does this mean, and how is it measured?

>>". . . restricted or unrestricted flow of electrons."<<

***What does this mean, and how is it measured?***

You'll need an electronvelocitormeter. Available at better auto parts stores, it will also enable you to adjust the proper level of headlight fluid in your car.

**I forgot to check my headlight fluid the last time I put water in my tires.**

You're poking fun at a very serious situation here. Recently an AC worm infiltrated my power conditioner rendering it useless. Before I became aware of that rascal, it had also infected my power cords totally disabling them. The copper and gold conductors had turned to lead. I'd advise all readers to install a firewall on your power conditioner and dutifully monitor your electron speed. Forewarned is forearmed folks.