Daddario NS510 E-Cello medium

The set is pretty well matched (although if you plan to add an extra E-string, it doesn't blend as well). I use these strings with NS-Design NXT5 electric cello, which also comes pre-fitted with these.

The strings are decent sounding but not spectacular. They give out enough volume so you can hear yourself when playing unplugged. When plugged in, it's mostly irrelevant anyway.

I would prefer a richer tone, especially as with electric cello you don't get the instrument body sound the same way you get with an acoustic instrument. These strings are a pretty good starting point, though. Especially since most brands are quite a bit more expensive.