slippy
Afaik 300-s are as unbrickable as 505 (means, one should really try hard to brick 'em).
According to boroda's (aka porkupan) manual, in case of recovery firmware, only 505's updater is available, here is a link:
http://projects.mobileread.com/reade...0%20Update.zip
it brings device (300) back into state from where you can try to reflash it again with proper prs 300 firmware using one of the boroda's packages.