A line transformer can be used. IME, not an elegant option, but it works. You are better off with a preamp that can drive the amp properly.
Impedance Matching Question
Most know that the input impedance of an amp needs to be ~10X the output impedance of the preamp for proper matching. Is there an inexpensive device that can be placed between a higher impedance preamp and lower impedance amp to help solve the issue? This is not for critical listening, but I do want it to sound good.