Do you have links for the software you used for re-flashing the device, and the firmware image(s) you tried to flash? It's possible the problem might be evident from studying these.
Also perhaps you could cut-and-paste the commands you used to reflash, and the exact error responses. The [code] tags for this forum are good for this.
It is unusual to totally brick a device to the extent that it needs physically hacking. Replacing the bootloader is a prime suspect when that happens. It's almost never necessary to reflash the bootloader. But sometimes the firmware image is bundled with a bootloader as well as the kernel and file system image. So the whole lot gets flashed in.
However, if that bundled bootloader is somehow incompatible with the hardware revision of the device (and it can only take a very minor change), it could still brick the device. At that point you do have to physically open it, attach a JTAG cable, and re-flash the original bootloader back into it.
As for which method to use - UART or JTAG .. I would try UART (serial) first, since its cheap and easy. A UART cable is only a pound or two (search ebay for pl2303). But if the bootloader is toast, UART won't work any way. JTAG cables are a bit more - £5 or £10 for a better USB-based adaptor, e.g. clones of the Altera USB-Blaster.
Cut and paste the exact commands you tried, and the exact reports. Maybe repeat exactly what you did before, and log everything to a file, and paste it here using the [code] tags.
Good luck!
cheers, a