I went from a Monolith to the PCU 13. As others have said it depends what you mean by 'worth the extra £1k'
Increases in performance of many things in life (inc av) are not a linear relationship to cost, twice the cost is often not twice as good, maybe only 20% better, depends how much you want that 20%
The PC13U is defo better than the Monolith, its biggest advantage being pure room pressurisation, ie the bass you feel in your chest. I feel my pc does this in a way the Monolith couldn't.
However, to this day I have nothing but respect for the Monolith. Given its 'small size' and low cost it gives stupendous performance, more than enough for 99% of people.
If space or money were ever an issue in the future, I would not hesitate to buy one again