I've used Ken Smith custom balanced string sets on my Kubicki Factor for years, but on the XL2UF, I've only played it with the D'addario ESXL170 set that it shipped with. I'm looking forward to experimenting with different gauges and brands.
The only GHS strings I found worthy on a Steinberger were Roundcore Boomers, Pressurewounds, Progressives, and surprisingly Brite Flats.
