biggest difference...speaker cable or IC's?

for horns: a networked-type speaker cable has worked out very well for me: MIT paired with classic Belle Klipsch.

Regarding conversion to biwire: although my MH750 Magnum's are biwire, I wouldn't necessarily expect a significant improvement in going from single to biwire with the same model of cable. Sometimes a switch to biwire is better, sometimes indifferent, sometimes even a degradation; you just have to try it.

So in this situation I guess I'd probably try some different interconnects, although not knowing about your equipment or present cabling leaves significant info. unknown (in terms of offering a worthwhile opinion).

A lot depends on which is longer. In my experience, where I have long speaker cables (25') and short ICs, the speaker cable has made bigger differences. Also, the purpose of balanced ICs is to cancel out the effect of the cable, so the balanced IC is probably pretty good, and not as likely to yield as much improvement.

Unless your current cables are somehow pretty deficient I don't know that you'll hear much real difference. For starters, I'll cast another vote for leaving the balanced ICs as-is - unless they're actually coathangers you're unlikely to find significant improvement there.

Bi-wiring? Well, if you're not going whole-hog and actually bi-amping, then that's probably not the answer you might think it is (unless you just wanna buy more wire ;~). And please don't take my opinion - you can read a fairly plain and pointed "what cables" page from a fairly popular purveyor of decent wire and see what they say at: Cobalt Cables. No, I'm not associated w/ 'em in any way - I just find it refreshing when a cable manufacturer tells the unadorned truth.

And buying cables to tame your highs? That's realy using wire to be a tone control, and that's the proper job of the electronics. If your pre-amp isn't doing the job for you there, then maybe it's time for a full-fledged equalizer - and you can get one heck of a higher-end equalizer for less that the price of a pair of MIT Magnum bi-wires. I have nothing against MIT or buying good wire, but I just think it isn't the job of speaker cables to color or change the signal - exactly the opposite. And maybe an equalizer isn't even needed here - maybe just tuning/tweaking your speaker placement and the room (treatments, etc.) could be a better place to start to fix those highs? Good luck!