I recently had the Klotz tonearm cable in my Rega RB900 replaced with the Incognito wiring harness, and I've been very pleased with the improvement. I'm not sure if this cable will work in your Sumiko arm, but you could contact Brit Audio to find out. Here's a link to their page for the Incognito harness: http://britaudio.com/Incognito.html
My other comment is to urge you to consider upgrading your arm. I, too, owned the Sumiko MMT arm, and used it briefly with the Grado Reference (4.5 mV output) cartridge. When I upgraded from the MMT to the Rega RB900, I was surprised at the degree of improvement in sound quality. The Grado Reference cartridge is an excellent unit, and to fully enjoy its capability it needs to be used with a top-quality arm.