The NiMH Imedion 9v's are excellent rechargeable batteries. Just make sure you get the 9.6v version for use with the nbox.
However, you need to be careful not to over-discharge them. The original nbox / reutelhuber uses a high current design, ~50 mA draw each from the two batteries powering the opamps, and does not have a low-voltage cutoff mechanism. As a result, it will easily discharge these batteries to < 1 v total, which is terrible for their longevity. Each 9.6v battery is comprised of 8 1.2v cells in series, so dropping the total voltage to < 1v is effectively lowering the voltage to ~.1 v per cell. Generally speaking, it is not a good practice to discharge a NiMH cell below .9v, so draining it down to .1v is about as bad as it gets.
I have 16 of these... The original 8 I bought 3-4 years ago have served me well, however I am now starting to see failures from over draining them. Some of the batteries now show physical changes (bulges, indents, etc.), but still seem to take a charge ok. Others are now completely dead and will not accept a charge.
If you're using the nbox platinum, the risk of over discharge is basically non-existent, unless you left them in the unit while turned on for several days / weeks. I doubt that they have added a low-voltage cutoff circuit. In comparison, the original nbox will discharge them to < 1v total in ~6 hours.