Concerning impedance, a 10megohm DI is great for Active and Passive.
Cause the signal = di box input impedance/(source impedance+di box input impedance)*100
So the higher the impedance, the better the signal you got (=the lower the voltage loss the DI box impedance will create). On the opposite, a 2 megohm is not so good for passive pickups.
Concerning the volatge :
"""""The only caveat with this box might be if you're running actives like an EMG... It might not be able to handle the loudest passages of your playing.... I.E. it might 'splat out' on the strongest transients.""""""
Which means that sometimes the EMGs ouput level can be so high that the Countryman input cannot take the signal. Its a 5 V input. And the very top of the signal is "cut"..
My 2 cents : Buy a Radial J48 first. Record, reamp, make few projects with it. Use it for passive and active. You'll get 85% pourcent of the signal with passives anyway. It is not so bad. And when you can afford it, buy a countryman for passive only.