The RUMBA board has a design flaw, which is that the mosfets it uses are not intended for 5V gate drive. Any particular board will probably be OK up to 11A heated bed current, but it is not guaranteed. Your heated bed with a resistance of 1.5 ohms will draw about 8A. So your RUMBA board can probably drive it directly. Try driving it directly from the board. If the heated bed mosfet gets too hot to touch, replace it by a proper logic level one with a low on resistance.
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