Ok, took the 300 and 500 to church. Both have some buzz, the 300 noticeably has more. Seems to get louder at higher volumes. Both do most buzz with the tube on. Found my old seems to get into creamy OD easier, which is not what is supposed to happen with the improved tube stage on the IVs. In a live setting I would hardly ever, if ever notice the buzz on the 300, but at home it's annoying and is likely going back. Really, really hate that. Sounded really good and I got a great deal. No one has one around here to see how normal my buzz level is. Not drastically more than the 500, but definitely more. While there I fired up the Ampeg, not the slightest sound at idle.
@TBird1958 Before I box it up, can you confirm yours is quite when unmuted and and a bass connected? Even so, what Chuck and Miles said just sounds right about sending it back. Hard to not follow such informed advise.