09-08-09: Almarg
To calculate the combined input impedance of the sub and the power amp, divide the product of the two impedances by their sum. Example: If the sub's input impedance is 25,000 ohms (25K), and the power amp's input impedance is 50,000 ohms (50K), their parallel combination would be (25 x 50)/(25 + 50) = 16.67K.
Perform this calculation for your MV-60 amp and Sunfire sub. The general rule of thumb is 10X. That means the input impedence that the preamp "sees" should be at least 10 times the preamps output impedence. The equation will give you the combined output impedence that your preamp will "see".
Personally, I find the 10X rule to be a tad low, I still hear some roll off. The higher the better. I find that at least 20-30X is better. Of course 100X is even better yet.