Optimal length for digital cables: 1m or 1.5m?

First, I'm not sure why you would be using a digital cable between a DAC and a preamp. Usually the left and right channel analog outputs of a DAC would be connected to analog inputs of the preamp, and the cables would be conducting analog signals.

In any event, assuming that the cable is conducting a digital signal, yes 1.5 meters would be a safer choice. See this paper for an explanation. Your chances of getting comparably good results with a 1 meter length are dependent on many technical parameters of the specific components that are being connected, many of which are not usually specified by the manufacturers. The particular cable type is also a factor.

So I would suggest that you indicate the make and model of the particular components that you want to connect, and that you do not attach much significance to experiences with 1 meter lengths that may be reported unless those experiences were with the same components.

Regards,
-- Al

If I may ask you a similar question but for the analogue signal xfer: assuming that there is no impedance mismatch between the preamp and power amp, how short can a RCA interconnect be?

If the goal is to minimize the sonic effects of an analog cable, the shorter the better. That may not always be subjectively preferable, though.

The technical factors that are involved in digital signal transmission, and in analog transmission at audio frequencies, are very different. The result being that length considerations for the two kinds of cables are also very different.

I should add, btw, that if a very short length is practical for a digital cable, such as 6 to 8 inches or so, that would often be a good choice. It is the intermediate lengths, that are significantly longer than that but significantly shorter than 1.5 meters, which stand the greatest chance of not being optimal.

Regards,
-- Al