How essential is shielding?

Capacitance of many typical Belden cables run in order of 25pF per foot while the best interconnects go as low as 3.5pF/ft thanks to use of expensive dielectrics (foam Teflon) and special geometries (oversized tubes). Cable with no shield eliminates hot-to-shield capacitance but hot to return wire capacitance exists and most likely is increased because unshielded wires tend to be twisted pairs. Twisting two wires exposes them to EMI evenly, working effectively as a shield (both for radiating and receiving) but unfortunately increases wire to wire capacitance.

Shielding itself is a very complicated business. For instance, shield made of non magnetic material does not protect against EMI (it cannot) but induced high frequency interference travels on the outside of the cable - a shield, because of skin effect. Add multiple shields + shield's inductance + twisting + additional returns and you'll get something nobody can understand. If it sounds good in your system go for it and don't worry about the science. Noise pick-up is system, and not the cable, dependent - all components participate.

It is worth to mention that EMI pickup of lower frequencies such as approx 500kHz generated by many class D amps can be picked-up as direct connection thru capacitance (important to keep wires apart or right angle) and not thru the electromagnetic pickup. It is because of lenght of the wires that would have to be hundreds of feet long to be any antenna for transmitting or receiving. Antenna works quite well at 1/4 wavelength but drops rapidly below that becoming practically ineffective below 1/10 of wavelength.

As for balanced being remedy - it works but problem starts at high frequencies where noise gets thru transformer capacitance (if used) or instrumentation amplifier (if used instead of transformer) rectification effect (uneven slew rates going up and down cause extraction of the modulation of the signal)

I have found it can make a difference in noise levels in some cases and not in others.

You have to experiment to determine.

Orientation and position relative to other wires with current passing through or powered devices with transformers that emit EM fields matters a lot.

What are your thoughts on (Synergistic´s) active shielded cables?

"What are your thoughts on (Synergistic´s) active shielded cables?"

Very high tech and very expensive.

I'd have to hear them to make any value judgement.

Personally, I prefer an effective shielding approach that is not active, like mu metal, which I already use to good effect to shield my phono step-up device and ICs from external noise inducing EM fields.

FWIW my Audioquest CV6 speaker wires use a similar but lower tech and cost active approach with their DBS devices. These speaker wires are very good but I have not been able to determine how much DBS has to do with it.

Of course, low level ICs and speaker wires are two different beasts as well to start with.

Hasse, Active shielding means placing buffered common mode signal back on the shield thus reducing capacitance and therefore pickup. It is technique widely used in low level amps like EKG amps but Synergistic's definition is different. They just claim putting battery on the shield. Battery with high voltage is used by Audioquest to polarize dielectric (dipoles) and make it sound the best even if not used for a while (no break-in). Synergestic is a little vague on this.