The main problem with connecting electric guitars is the influence of the resonance peak of the pickups by complex loads. Some authors have developed the relevant basic principles: e.g.: Dipl.-Phys. Eberhard Meinl: Elektrogitarren VEB Verlag Technik Berlin 1987 and Helmut Lemme. It would therefore be good if the complex load were very low. So cables should be as short as possible.
That would be so if the goal was to achieve the best HF response, however it's not the case. There have been many attempts to increase the HF respnose of the electric guitar, in a holistic way, i.e. considering all elements of the chain, the pick-ups, the cable and the amp.
For a start, for a reason that maybe both physiological and acquired taste, frequencies above 4-5kHz are not welcome. Typically, most guitar amps use a 10 or 12" loudspeaker which resricts the HF response. Guitar amps with tweeters have been known to exist, and were never popular, often considered too shrill.
Pick-up designers have come with low-impedance designs that could potentially shift the characteristic frequency of the pick-up/cable combo. In the lata 60's, Les Paul did that in his guitars and Gibson incorporated these designs in four models, that were a commercial failure. Their current value is only historical, not playability.
A very easy experiment can be done by connecting an electric guitar to a typical stereo, or to an electroacoustic guitar amp, which has a much extended HF response.
A look at amp simulators (boxes that allow connecting an electric guitar to a standard PA) shows that they all include a low pas filter at about 4-5 kHz, in addition o whatever EQ and processing.
Guitarists that swith from cable to RF transmitter almost always need some time to accomodate the increased HF content reaching their amp(s), because of much lower capacitive load of te short cable from guitar to transmitter.
Many transmitters include a "cable simulator", in the guise of a switch that selects different values of input capacitors.
Microphone cables: Ing. Wuttke (Schoeps) gave a talk on this at a sound engineers' conference years ago. The output circuit and the output resistance of the microphone are of considerable importance for the influence of the complex load on the subsequent cable. The best way here in my view: put the microphone preamplifier as close to the microphone as possible in order to minimize the complex load.
I'm not sure the answer is as clear cut as it, considering there are two actions at work.
One is the capacitive effect, the other is interference noise.
Regarding capacitive effects, you have a low-pass filter constituted of the microphone impedance and the capacitance of cable C1 of length L1, and an other constituted of the preamp's output impedance and the capacitance of cable C2 of length L2.
If the output impedance of the preamp was identical to the microphone impedance, the best location for the preamp would be at mid-distance, with L1=L2.
In practice, the optimum depends on the relative value of the microphone and preamp output impedance.
For example, Schoeps microphone has a rated impedance of 42 ohms, which is likely to be less than the typical impedance of most preamps (50-100 ohms).
In that case, and considering only the HF response, the best choice would be to put the preamp at the other end of the cable.
The other factor to consider is global noise.
It depends very much on how much noise is induced in the cable, but in many cases, elevating the signal by placing the preamp close to the mic will be enough to increase the global S/N ratio.
It's clear that the choice must be balanced with other factors, such as availabilty of power and distant gain control possibilities.
