Makerbot Replicator 2 restarts after preheating the hbp
Clues so far:


You changed your hardware to add a heated bed.
You've had the same symptoms with three different firmware versions.
Your Replicator 2 reboots right after preheat, regardless of temp set.  


My Assumptions


You have not tweaked firmware or other settings that could cause both the heated bed and hot end to draw high loads simultaneously.  (By default Makerbot handles this in firmware, avoiding situations that could over-tax the 9.2A power supply which isn't really enough to handle preheating both at the same time.)


Potential Problems: Listed starting with the easiest to fix...


No problem, carry on: If you are printing from the USB cable, a restart after preheat could be a normal condition.  The Replicator 2 reboots to reset inputs when you start a print over USB...this is normal.  So, if you're preheating with the buttons and then you start a print, this would be expected behavior.  Try printing from SD card and it shouldn't reboot.
Power Supply voltage switch: Make sure your power supply voltage input switch is on the correct voltage, either 110 or 220.  Having it set to 220 when you're using 110 can cause reboots due to insufficient power. (brown-out)
Wrong heated bed: If your heated bed draws too much power (it is higher power than the Replicator 2 can support with that power supply), you may get reboots due to insufficient power. (brown-out)
Short somewhere: A short somewhere in your system could cause too much power draw.  This could be somewhere in your heated bed, your hot end, or other.  Inspect carefully around the areas where you moved wires.  You could have a frayed wire contacting the frame, a screw shorting between two wires, or a little stray "hair" of copper wire sticking somewhere it shouldn't.  This too will cause a reboot due to insufficient power. (brown-out)


I hope this helps.  For further reference, read this thread as I believe it has some relevant info.  :-)