I will second the Pure Note Alluvion. It is exactly at the upper limit you refer (at least for now). I recently bought the balanced version to use between my W4S DAC2 and W4S ST-500. Still burning them (currently I am at about 150 hrs) but these are remarkably smooth yet have such an astonishing amount of internal detail. Look for comments at other forums too. The music beatifully floats between my Martin Logans with exceptional soundstage and absolutely no "silverish" sound or glare. At least in my setup you can nearly feel the limits of the recording room. My other balanced ic's are a Tara Labs Master Gen2 and a DH BL-1. For comparison the Tara (still use it in my Theta gear) is no slouch. It is smooth and very musical but not that resolving, the DH is also a very good cable to startup and might fit well in your system, but they didnt seem to have the inner resolution and smoothness of the Pure Note (of course they are made of different materials and at different price). Alluvion is highly recommended even if they were $600.
Best budget $300 Balanced IC Recommendation
I have a Cambridge Azure 840C with both Balanced and SE outputs. I have been using Kimber KS1021's with Black Pearl silver and copper stranding. Anyway, I wanted to try the balanced connect and ordered a pro cable for about $20. I could not believe the results! The pro XLR cable sounded almost as good as the Kimber and it's not even broken in! This tells me balanced is the way to go. I'll probably sell the Kimber's to finance the purchase or trade for a balanced IC. Looking for some help and suggestions - thanks!