I've had a low-end Sony HT in a box for a few years now, and recently I have noticed really bad noise/distortion in the range of human speech and singing. Because it is across all the speakers (including 2 Magnavox speakers that I took off of an old boombox) I assume that the problem is with the receiver. Is there any way to test this, or should I just bite the bullet and get a low grade receiver while I patiently try to accumulate enough cash for the new Marantz separates (AV8003 and MM8003).
The receiver is model STR-K670P if that helps any.

tl;dr There is distortion in the range of human voices and its my receiver's fault. Is there any simple fix?