Help with Fender Champion "600"

[stevel]

This is one of those relatively new jobbies with the two tone brown/beige tolex and the "suede" grill cloth.

I like this little amp.

But something's "hissing" in it.

It happens on certain notes, especially when played hard (and not that the amp's breaking up yet at that point).

It's relatively quiet, and in the background. If you weren't listening intently you probably wouldn't notice it (but now that I notice it, it bugs me).

Like I'll play an E on the 7th fret 5th string and as the note dies away I'll notice this hissing in the background, that dies away too.

Is it likely the speaker?

I think I could rig a cable to come out of it's output jack and on to the terminals of the speaker in a Pro Jr. I have - is that OK? The Speaker Out put says 4 ohms minimum at 5 watts.

If the Pro Jr is 8 ohms can I test it with that? (I'm assuming the Pro Jr's speaker is more wattage, though not sure about the resistance).

Likewise, could I plug in the Pro Jr's extension jack and power the Champion 600 similarly, just to test it?

TIA,
STeve