I wouldn't chance my iRiver to 6V. Maybe it works, but many iRivers have been fried from using the wrong power supply, and I intend to make sure mine doesn't join that pile.
I have a device which you plug into your car's cigarette lighter, and it exposes a standard USB jack like on a computer, which is 5V. I don't know where I got it, someplace on the internet for about $3. Then I have a homemade cable where I cut off a USB cable and soldered on a iRiver sized plug, and it works fine. I can't tell you how much charging power it puts out for sure. I typically record to an H120 (or Korg MR1), and at the end of the night the battery is running low. I use this thing to power the iRiver so I can listen to the show on the way home without the battery dying. I've used it more for my Korg MR1 than I have my H120, but I'm pretty I have used it for the H120.