Yeah, not even sure where I'd get an 8 ohm 70s McKenzie Speaker
The difference is pretty amazing... Especially the low end, which from the amp is really flabby and loose... from the cab it's super tight...
I'm guessing the cab you have is closed back?
Putting a new speaker into the amp itself will give the same flabby results in that case.
It's down to the amp/cab construction. Open backed designs allow for quite a bit of phase interference, especially in the lows. Some frequencies get nulled, others getting unnaturally boosted. Some say chewy, others say flabby.
Good for blues and classic rock, not so hot for modern rock, funk or metal.
From memory the valves hang down into the speaker area in that amp so closing up the back is not really an option as you'll be frying valves regularly.