Speaker impedance changes with frequency -- so it probably drops under 4ohm. Low impedance = more difficult load. This said, yr transistor Cambridge should have no electrical trouble making music to about +8-10db(spl) over whatever nominal sensitivity rating these Polks have. Mind you, this is just a rule-of-the-thumb estimate.
So, if the spkrs measure say, 85db spl nominal/W/1 metre you'd go to ~95W peaks w/out clipping and have some extra energy left in the Cambridge to spare... not very loud overall nor extended dynamic capability...
...I seem to remember these spkrs aren't very sensitive -- why not also consider other simpler loads. Many around. CHeers
So, if the spkrs measure say, 85db spl nominal/W/1 metre you'd go to ~95W peaks w/out clipping and have some extra energy left in the Cambridge to spare... not very loud overall nor extended dynamic capability...
...I seem to remember these spkrs aren't very sensitive -- why not also consider other simpler loads. Many around. CHeers