The upgraded tk-566 seems to have a max input voltage of 4.2V. Does this means that it is now possible for mount 2 extension tubes and get a 3xAA nimh with 60% more runtime (not 50% because the driver should be more efficient at 3.6V than it is at 2.4V) ?
It should work and you can get extensions for a reasonable price at Manafont. But the form factor is a little odd to hold because of the weird thickness/length ratio with 2AA and very odd with 3AA.