You can try fiddling with this:
VCDS under radio and coding you can find bit 5 which is eq. (might be different bit's for you but you'll find it)
You can problably choose presets and try out if any eq makes it sound better for you. (you can do it live while radio is playing, it will squeak after saving settings and then resumes)
And yeah.. this is tiguan and RCD/RNS has preset eq-s only for skoda.... it's like VW engineers just slapped that piece of * into dash and didnt give single thought how it will sound.
I'm using linear and I have selected "vw soundsystem or dynaudio" settings because I have external amplifier. And I still had to use high level in on my amp because selecting vw soundsystem or dynaudio will not activate nor convert speaker cables into RCA outputs!!! it marely drops volume from 24db to 12db.
So.. VAG also runs their high end dynaudio garbage signals over hi levels :mad2:
though VW and almost all oem headunits are total pile of garbage, best upgrade is to throw it out of window and install some cheap pioneer headunit without nav and cd - you will get 10x better sound!
also how ever you choose eq, sound system or no soundystem, or replace speakers to 2000€ ones, oem headunit will still **** up sound and it badly messes with rear speakers.. even linear eq, rears sound flat and dull.
about sdcard, you need to use fat32 format and wav or mp3.
mp3 320kbs or lossless wav but it takes up alot of sdcard memory.
also 32gb is max that this garbage headunit can support
but in the end, basically sorry that you upgraded rcd to rns which are same except nav and some other stuff.
better option would have been random alternative headunit with it's own camera in system. (also 10 times better camera image that can adapt low/bright light situations like pioneer)
some day I will upgrade to this:
http://www.pioneer-car.eu/eur/products/mvh-av280bt