Just to keep you updated, I've fitted a pair of Celestion Truvox 1225 8 ohm speakers in the cabinets - funnily enough, the one that was REALLY! bad already had an 8 ohm driver, of some unspecified make! - the other was a Celestion 4 ohm, presumably original?.
Having done ONLY that, I gave them a brief test, and they sounded pretty good like that - as I'm struggling getting hold of suitable inductors, I might leave the bass crossovers alone, it will only give a bit more mid-range to the bass units. I've order some resistors for the tweeters, but I wasn't able to locate 6.2 ohm ones, so I've ordered 4.7 and 6.8, I'll try one in each and then compare them - see which I prefer the sound of.
I crudely 'measured' their overall impedance tonight, simply feeding them through an 8 ohm resistor, and monitoring the signal on either end of the resistor with the two beams of my scope. I simply set one beam to twice the gain of the other, and slowly swept a sinewave up the audio band. I was pleased to see it never dropped appreciably under 8 ohm, even without the resistor change - most of the range was just over 8 ohms.