Good news. I always tighten the nuts up each time I change strings on my guitars, as they do become loose with vibration. Worth checking them every 6 months if you tend not to change bass strings that often.

The kit strings are awful. Almost always low output, quite dead sounding and have been known to sound pretty uneven from string to string. Only really good for an initial set-up. If it sounds good with those, it should sound a lot better with EBs on.