Do it! Upgrade the preamp.
I suspect that making use of the amp's balanced inputs would only be realized if you use a truly balanced (+/- phases) from your preamp. To add an extraneous transformer or phase splitter circuit, like so many products do, to achieve the 2 phases is only going to cause loss and coloration.
I have often found the preamp (line stage) to be the far more critical link in the system than the amp, and even the speakers, when it comes to the magic and musicality. You have put so much money into these areas but severely compromised this all important preamp stage.
If you're looking for a truly balanced preamp, line stage and or phono, and you like the ARC sound, the best choice would be to find an LS5 MK II, which has 30db gain and is often for sale in the $1700 or so range. Every now and then the matching PH2 is available and this is a killer unit. I just sold mine to change to the BAT VK-P10 and these two units were so very very close.
Peter, I have been listening to the ARC VT100mkII with the ARC LS 15 for about four years. They compliment each other quite nicely, especially after you replace the four Sovtek 6DJ8s in the preamp and the eight Sovtek 6DJ8s in the amp. The Sovtek may be a good sounding, reliable tube, but it just does not image or have the subtle inner detail or dynamics of your better Amperex, Siemens, Telefunken or Mullard 6DJ8/6922/7308s. Further, the Svetlana 6550 power tubes are a bit shy in classic tube warmth and liquidity -- the Svetlana KT88s are an economical and quite satisying upgrade. I would recommend my previous post on the difficulties I encountered re-tubing this amp -- no need for you to blow up handfuls of tubes and resisters like I did. In addition, you will never experience what the VT100mkII is capable of until you replace the stock power cord.
The LS 15 does not have a detachable power cord, nor does it have a phono stage. I use the EAR 834P. This phono stage can be matched with very low output cartridges --- my Benz Micro Reference is rated .33 mV -- with excellent results.
As to interconnects, I find the ARC gear prefers silver cable to copper and balanced to single ended (all cable being equal). I am presently using Silver Audio Appassionata balanced IC between the preamp and amp, and Zen Accoustics Silver Reference single-ended IC between the preamp and Cary 303/200CDP. Both are more revealing and dynamic than the Cardas Golden Reference IC that I use between my phono stage and preamp. With your gear and your preamp budget, you will want to have IC cable that cost in the neighborhood of $500. You might find a $250 cable that's 85% as good and will serve your purposes. But why buy these wonderfully musical components if not to enjoy what they are capable of?