I did find one website that had them in stock, but the problem was that they only had two of them, looks like I'll have to do some serious googling for other suppliers, I tried Evatco and they didn't have any in stock whatsoever, I like 10 inchers for Bass too, it means that the cab will be compact enough to put in a car, yet still deliver a good Bass sound.
I'm going to be wiring the 4 new speakers in a series-parallel configuration, so if I'm aiming for 8 Ohms total impedance then I need 4 X 8 Ohm speakers, although my Super Twin can work with a minimum of a 4 Ohm speaker load connected to it, the Super Twin would then be putting out it's maximum power at full volume, 180 Watts RMS or 395 Watts Peak (on instantaneous transients, like pick attacks), well that's my interpretation anyway.