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

the length that 'supplies' sufficient rated impedance.

Mea culpa Al! I meant between my Cary 306 that I use as transporter and the Eastern Electric MiniMax DAC Plus (which fortunately has the BNC connector as well). Thanks for the link.

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?

I agree with AL. A friend of mine and I have a/b'd a 1 meter Shunyata Zitron Python digital cable and a 1.5 meter Moray James cable and we both preferred the Shunyata cable but we both understand this does not answer your fundamental question. I faced the same cost conundrum you are considering and opted to spend the extra money elsewhere in my system. The only way to know for sure is to compare the two lengths of the same cable with the same equipment, IMHO.