The nyckelharpa sounds so much better in tune. But ... it doesn't actually work very well if it's not! So once it's in tune it's a happy instrument and will sing and ring, but let those sympathetic strings go out and it'll get grumpy and quiet. This is how my nyckelharpa is tuned - but there are several different tuning systems so be careful and find out what yours is! Below is a diagram that I hope helps!
Enar Magnusson Tuning System
If you have an Enar Magnusson nyckelharpa the sympathetic strings are organised differently. Here is a video to help with that!