Compared to Peaveys 5150 a Jubilee can sound "boxy" in terms of nasal, when the mids are turned up past 6.5 or 7 because of that extremely effective tone stack. The 5150 also may sound more "pleasing" to the ear, because it has a more "creamy" overdrive/harmonics character, and a deeper fuller bass. When you turn up the bass on a Jubilee, after around 8 on the dial, a pretty noticeable low mid boost kicks in, which can also be associated with boxy. Maybe the buyer is used to play with karge amounts of bass, using the 5150's Bass AND resonance controls. To achieve the same level of bass, he might have turned up bass past 8. The buyer should make a video of both amps and give you the settings of either. Just that we can compare. Might just not be something defective, but someone not used to the sometimes finicky tone stack of the Jubilee.