Dpe,
Fortunately most of the Cardas cables have very low capacitance, with the Golden Cross at 25.5 pf/ft, and the Golden Reference at 12 pf/ft in single-ended form (and lower in each case in balanced form, but it looks like the LA-150 only has unbalanced outputs).
25pf/ft x 20 feet (slightly more than 6 meters) = 500 pf.
The capacitive reactance (impedance) of 500 pf at 20kHz is
1/(2 x pi x f x C) = 1/(2 x 3.14 x 20,000 x (500exp-12))
= 15,924 ohms.
That is very comfortably higher than both of the preamp's selectable output impedances. It can be expected that the 400 ohm and 1200 ohm output impedances will vary significantly with frequency, but I suspect that they would only rise significantly above those numbers at deep bass frequencies, where cable capacitance is unimportant.
So I think you will be fine as far as cable capacitance is concerned (with either of those cables), and just go with whichever output impedance sounds better.
Although keep in mind that if your power amp's input impedance is on the low side, meaning in the range of 10K to 30 or 40K or so, the 1200 ohm setting (which probably rises considerably above that number at very low frequencies) could result in some attenuation of the deep bass. That would occur regardless of cable length. If your power amp's input impedance approaches say 75K or 100K, then I'm sure the 1200 ohm setting would be fine in that respect.
As you may be aware, that is sometimes referred to as the 10X rule -- power amp input impedance should be at least ten times greater than the output impedance of the preamp at the frequency for which the output impedance of the preamp is highest, which is usually at 20Hz, especially in the case of a tube preamp.
Regards,
-- Al
