There's a reason they are known as Oxfarts. They were built with looser tolerance in the voice coil gap and they tend to distort on transients. Fenders are known to be a bit farty anyway when cranked up, particularly with the Bass turned up much at all. Combine that with the Oxford tendency to distort transients and oilla, Oxfarts.
My experience is with the Oxford reissues in my '63 Vibroverb Reissue...
Oxfart is about a perfect description. At lower volume, they didn't have much in the way of highs, but a lot of midrange. They sounded nice at low volume... they were very inefficient, so 40 watts was a good practice amp. But the minute you got the amp around 7, they were unusable. They farted out big time and sounded inarticulate... I was afraid they would shake themselves apart!
Yeah, the trouble is those amps weren't designed to be turned up past distortion so when rock and roll came along the speakers just weren't up to the task.
If you have the originals, pull them and put them somewhere safe and get yourself a pair of modern speakers that can handle the power and bass.
WHat those are is up to you since it is so subjective but a lot of guys like the Jensens or Webers on the lower powered blackfaces.
The Oxfords from the brown era (not reissue) are pretty nice. They compliment the circuits well. But the blackface era on are pretty ****** IMO. I gig a '74 VR that had them stock and I couldnt wait to swap them out, and I rarely mod or change stuff like that if nothing is broken.