The PRS-900 screen is "supposed" to be better than the PRS-600.
So I'm wondering if you might have a bad unit.
On the other hand I wanted to return my 600 when I got it home. It was VERY clear in the store (multiple overhead light sources). But I like to recline, feet up, and read with a 60 watt floor lamp behind and to the right.
The contrast (no pun) between the store lighting and what I use at home were VERY striking.
Add to that the fact that my first ereader had back light (the EB1150) and I wasn't happy.
The 600 however suits me to a T, and I soon got used to it being darker.
My advice? Try it for a few days or a week. If you can't get used to it, return it. There are some readers that have back light, and they are cheaper than the Sony.
But it suits me and I wouldn't trade it in. At least not this year.
P. S. - LED lights - try a cheap LED light before buying a "good" one for reading. After trying one myself, I returned it. Use a brighter bulb in your lights or increase the font size.