As a practical matter here is what I have learned. These are newbie level lessons but help me with thinking about design and construction of high quality audio, and they have served me well. Please forgive these if this sounds like "baby talk" to you.
1) Worry about interference on HIGH IMPEDANCE or HIGH RESISTANCE circuits. Think of it this way; when the wire has high resistance... the surrounding air (and any signals therein) start to look like a good path for an electron (this is from the "think like an electron" school of thought").
2) Length matters, wire is wire - There is nothing inherently different between a circuit board trace, a wire, and a resistor lead, or the wound foils in a capacitor or the pins on a potentiometer. The differences are the presence of shielding (ground planes, twisted pairs, shielded cable) and capacitive coupling (between traces and ground plance, pairs of twisted wire, shielded cable, etc) or coils that can create inductive effects (like the wound foil in a film capacitor). So as you consider your design, try to minimize the length of high impedance circuits, and you will minimize the effect of both interference, and unintentional filters (a high enough impedance creates a filter in the audio range even with very low capacitance/inductance). So within your EQ for instance, some pots will likely be on higher impedance circuits in order to achieve frequency control with smaller inductors and capacitors. If you were really worried, you could focus your attention there.
3) Cable impedance is easy to calculate, (spec'ed on quality cables), and are measured in pf per foot. It is also pretty easy to measure if you have a DVM that measures capacitance. Just take a bit of cable (use a long bit to get accuracy) suspend a few feet of it in the air twisted or however you intend to run it, and measure the capacitance between the open pairs. In the case of shielded cable, measure between the shield and the conductor. In the case of multi conductor cable, there are different capacitances between conductors, and between conductors and the shield, dependent upon how they are wound, and the insulation thickness etc. Then divide the capacitance you measure by the length in inches to get the capacitance of your 1.5" cable. That will be a very very very small capacitance.
4) In design, it is nice to keep an eye on everything in the search for perfection, but boring old case size is often more effective that sophisticated shielding and such. Distance is natures mu metal! In general the force of the field decreases as the square of the distance
http://en.wikipedia.org/wiki/Inverse-square_law#Light_and_other_electromagnetic_radiation. This means that if you can move your transformer or inductor twice as far from the 1.5" wire, that you will decrease interference by a factor of 4! If you have a wire running 1/8" by some inductor, and move it to 1" away, by routing the cable better (wire ties and tie downs) you should reduce the effect by a factor of 64! (8 squared). If you have to make the wire a little longer to do that, you win big in the tradeoff.
5) Easy things to consider. 1) Separate the power supply to a different case, and run only DC into the box. Failing that... if you don't want to do it.... Make the box deeper or wider, and move the transformer to the far corner, keep AC wiring SHORT on both sides of the transformer and twist the pairs of AC conductors carrying opposite voltages (Twist the mains pair, and the secondary pairs). Keep both of those twisted pairs short, keep the power inlet and the power supply near the transformer. Use a toroid. Raise the toroid off the bottom of the case a little if possible. 3) Filter the AC line coming into the case, to avoid a high frequency entry point. 4) Pay attention to grounding, in a consistent way (read up on grounding read the Bill Whitlock papers on the Jensen site) 5) Twist AC and balanced pairs without their ground... and unbalanced conductor with it's ground. Pay attention to studio wiring too, connect equipment with balanced cables, and shorter is better (3 foot not 50 foot).
1.5" wire is very short in the scheme of things. In my shop I would probably twist it as appropriate but I am willing to bet that if you take 4 steps back and say where is my biggest problem going to come from... your target should be elsewhere.
Forgive me if I have added uselessly to the noise, there are far more experienced folks here than I. And I have been reading on the site that people want us newbies to shut up. But these ideas have served me well.
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