Searching on Familysearch.org, there seems to be one Carl Guhr, listed as a "gentleman," who lived in Wisconsin, born in Prussia in 1794. Lived in Wisconsin from at least 1860 to 1870, recorded in both censuses. No other Carl Guhrs found, and since Wisconsin is close to Chicago, I would gather that would be him.
In 1860, he lived at (and also in 1870, it appears)
Newton, Manitowoc, Wisconsin, United States
with whom I would guess to be his son August, a Farmer, also born in Prussia.
You might have some luck looking for descendants in Wisconsin and consulting them about Carl Guhr, there are multiple listings for "Guhr" in the Whitepages for the state of Wisconsin.
He died in 1881, per findagrave.com
My guess is that the fellow who wrote about Paganini is the same fellow who made your fiddle.