Nonetheless, try switching to a different EQ preset anyway, just to rule out the possibility of software corruption of the EQ data.

Even a flat EQ setting is still a setting stored in the dynamic data partition. The only difference between a flat EQ setting and a carefully tweaked EQ setting is the individual values for each parameter. They're both stored the same way and can both be corrupted. And, if I'm recalling correctly, a corrupted EQ setting can (a) result in unexpected strangeness such as crackly intermittency, and (b) can be set independently for the left and right channels.

If you do that, and still don't get any joy, then yeah, you're talking hardware fault of some kind that Stu needs to look at.