Ive got the Ks4 in the tuning package that Phormula do, not cheap but its a nice gadget

http://phormula.co.uk/ProTuningSolution.aspx

Manaul set of det cans is all you really need, but this is certainly a nice to have.

I use the headphones from a knockbox instead though as I prefer them to the phormula ones, they're good quality but dont insulate outside noise as well as the knockbox ones.
Cheap option that also works REALLY well is in the ear headphones, I had to use some when det checking a car on track (had to wear a lid) and was amazed how well they work so I sometimes use them just cause they are so easy to carry.

On my nova I was using the 0-5v output as an input to my Autronic so I could datalog and monitor knock, that was really good, but still I think your ears HAVE to be the thing you rely on.