Why do I lose the low end when using Jack inputs 1 & 2? Although it comes back if I press Hi-Z
When using super analogue channels with unbalanced line signals, it is important that you use the correct type of cable and jack connector.
Balanced connections allow the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference. A typical balanced cable contains two identical wires, one carrying the signal in phase with the source, and the other carrying the signal out-of-phase with the source. These two wires are twisted together and then wrapped with a third conductor (foil or braid) that acts as a shield In order to be effective, these cables must use three-conductor connectors on both ends which are usually XLR or Jack connectors.
By using a balanced cable from an unbalanced source you are effectively adding the signal on the in-phase wire to the same signal on the out-of-phase wire. Therefore, by the time the signal reaches the balanced TRS input of your “six” the signal is out of phase with itself, and the most obvious characteristic of this is that you will lose all the low frequencies. Pressing the Hi-Z switch effectively makes the TRS input of the “Six” unbalanced so whilst the problem disappears, it does mean that the pre-amp may not be best matched to the source in terms of it’s impedance.
If you’re using a guitar or a synthesiser, the output it’s almost certainly going to be unbalanced. Therefore you would need to use unbalanced cables with unbalanced 1/4’ Jack connectors. These are often referred to as “mono” or “TS” jacks and can be identified with a single black ring approximately 7mm from the tip of the metal connector.
However, if you are connecting from a computer interface (such as an SSL2 or 2+) the outputs are almost certainly going to be balanced and you should therefore use proper balanced cables with the correct balanced Jack connectors. These are often referred to as “Stereo” or “TRS” Jacks and can be identified with two black rings approximately 7mm (and 15mm respectively) from the tip of the metal connector.
Besides these examples, there are several combinations of balanced - unbalanced source / cable / connection configurations which will cause the same issue so it is recommended that you always use the exact correct type in order to avoid problems.