So the offset is actually 166khz and not 250khz as I've seen suggested? And it may be worth plugging in my signal amplifier after all?
The 166kHz is the step size of the tuner (their are other step sizes but thats the most likely for a cheap tuner). When tuning, the tuner frequency will be = TunerBasefreq + (StepNo * 166kHz). You'll notice that this means the tuner cannot actually set to many frequencies exactly but thats ok because the AGC circuits within the tuner can usually pull at least +/- 1 step size (or +/- 166kHz in this case). StepNo is simply an integer (usually between 0 & 4095 or 8192, depending on the step size)