I don't know that the strings would make a noticeable difference unless they're old - easy way to find out though!
Might be an obvious suggestion but have you tried adjusting your amp a bit? I hadn't really noticed any middle lacking from my HH, but it's taken a bit of fiddling to get a sound I like out of the rhythm circuit.
You're right though, the pick ups are somewhere between humbuckers and single coils, bit weird but at least they're quite unique! 😁
And if you decide you don't like them at all, save up and stick a couple of Seymour D SH3 stag mags in - http://www.seymourduncan.com/products/humbuckers.shtml
and then you can split the coils and have 2 quality humbuckers or 2 proper single coils at the flick of a switch! Perfect.
