Hello Zohra and welcome to the Cow Audio Forum.
Drop the Nagra and consider a Sound Devices 744T. If you have no more than three simultaneous sources, and you don't need to iso them, drop the 552 with it's two channel recorder and get a 302 and a 702.
It sounds like you may be past this, but..
http://web.mac.com/tyreeford/Site/Ty_Ford_Audio_Bootcamp_Field_Guide.html
Having good gear is important. Knowing how best to use it is imperative. Having the right mic two inches closer that you think it should be can make all the difference in the world. Good audio is not trivial. Unless you know, you don't know. :)
Regards,
Ty Ford
Want better production audio?:
Ty Ford's Audio Bootcamp Field Guide
