Speakers are generally designed to produce the desired response when power is supplied from a source that has a low impedance and flat response. Take a good SS amp, and connect the speakers to it through 10-20 feet of 12 or 14 gauge cable, and you meet that need very nicely.
At low and medium frequencies where flat response matters the most, a good SS power amp provides a source impedance of about 0.1 ohm or less. 20 feet of 14 gauge adds another approximate 0.1 ohm.
How effective a certain impedance is at maintaining flat response at the speaker terminals depends on the minimum impedance of the speakers. Worst case 8 ohm speakers might bottom out at 6 ohm impedance. Similarly, worst case 4 ohm speakers might bottom out at 3 ohm impedance.
Going back to our example involving 20' of 14 gauge wire and a good SS amp, the ratio between 0.2 ohms and 3 ohms (worse reasonable case 4 ohm speaker) is 15, while with the corresponding 8 ohm speaker, its 30.
I previously said that this ratio is better when higher and needs to be in the range of 10 (minimum acceptable for high quality audio) to 100 (maximum that could possibly make any improvement). IOW anything under 10 could adversely affect the frequency response of the speaker, and anything over 100 is overkill.
Now lets go to the shorter of the OP's cable runs - 100 feet of 14 gauge. Five times longer, 5 times more resistance or 0.5 ohm for a total of 0.6 ohms including the power amp.
This drops the ratio of minimum impedance to total source resistance to 5. This is like replacing a good SS amp with a cheap sloppy old tubed amp from the 1950s. I look at the rest of the system and I see a high end receiver. I don't think that the people who paid the big bucks for the high end receiver want to see its performance degraded to that of cheap sloppy old tubed amp from the 50s by the wrong speaker wire.
We then go the longer of the OP's cable runs - 400 feet of 14 gauge. 4 times longer, 4 times the resistance or 2.0 ohms for a total of 2.1 ohms. The first thing we see is that a significant portion of the power of the amplifier is being lost in the speaker cable. If we were using my example 4 ohm speakers with this situation, a something like -5 dB frequency loss would also affect the SQ of the speakers, but only at certain frequencies.
Again, I don't think that people are using high end speakers and amplifiers to have a speaker cable eat up 1/4 or 1/3 of their amplifiers power, or to add an additional 5 dB worth of response variations to their speakers.
I've suggested a number of ways to address these problems, only to find that the *real* problem was a receiver that screams and wails when attached to these long lines.
Attention now shifts to the screams and wails, not additional random coloration of an expensive system. Besides the losses in the long cables, they also have additional speaker cable capacitance, and there is a possibility of unintentional grounding or signal feedback problems. The OP is now trying to address that far more serious problem.
I don't think that the additional capacitance in the speaker cables should be a general problem that causes equipment to scream and wail. With normal two conductor cables, the cable's inductance buillds up faster than the capacitance. As a general rule, more inductance makes amplifiers more stable, not less stable.
However, this system should have never screamed and wailed when first hooked up, so what *should* happen is not the issue. Something odd is going on here, but it is the sort of thing you usually have to be on-site and run a number of experiments to isolate.