I agree with what has been written so far. I chose the M80s for a room almost the same size. While I will never use them to their mazimum from an spl perspective, the extra detail and bass these speakers deliver is well worth the price even for my size room.
On soundproofing the furnice room, it depends on how low a frequency the noise from the furnace is. If you have space, 4" of high density fibre board or fiberglass board is the minimum you should use to get good absorbtion down to 100Hz.
good luck and keep us posted as you progress.
