Assuming the speakers are the Zensor 1 and assuming, since you have a 5.1 system, you have an AV Receiver, we consider the following -
Dali Zensor 1 -
Frequency Range (+/-3 dB) [Hz]: 53 - 26,500
Generally you want the Crossover higher than the rated low end of the speakers. That takes some of the strain off the Front, and pushes it onto the Subwoofer. Assuming the system is set to SMALL, though I can't imagine any other setting for these speakers.
Next, at about 80hz Subwoofer (and speakers in general) move from Omni-Directional (all directional) to more directional, and the directionality becomes more pronounced as the frequency increase.
So, for most Home Theater situations 80hz is somewhat ideal.
Setting the Crossover to 60hz puts more strain on the Front Speakers, though not quite as much as running them full range. That strain could show up as more low end distortion. As should be clear, raising the Crossover also improves the Power Handling of the Front speakers, as most of the energy is in the lower frequencies.
Now if 60hz sound better to you, just make sure Front and Sub are set to 60hz. That's fine, if you like it. It is after all your system, and what works for you is all that counts.
Going in the opposite direction and setting the crossover to 100hz takes even more strain off the Front speakers but also allows the Sub to become more directional. Directional and non-directional, relative to a Subwoofer, has to do with Point Source, in other words, whether the Sub is coming from THERE, or whether it is coming from everywhere. The higher the frequency the more localized the Subwoofer becomes.
Generally 80hz is always the starting point, assuming the Front speaker are rated below 80hz. But, assuming it is still within the working range of the Front Speakers, if 60hz sounds better to you, then more power to you. Just understand how it works and what the trade off are.
For what it is worth.
Steve/bluewizard