Do I need to buy the external crossover and amp if I buy the M6 or M5?
No, the M5 or M6 is a stand alone monitor... that CAN be combined with the B5 or B6 bottom section ( which consists of a 12 inch woofer in each B5 pedestal or two 12 inch woofers in each B6 pedestal section ) when combined for a full range speaker they are then called a T5 or T6, This configuration requires the NHT crossover unit and amplifier for the woofers...here is a link for the T5:http://www.adnm.com/nht_t5.htm
But the monitors ( m5 or m6 ) as mentioned can be used without the subwoofer base's ( b5 or b6/ crossover/amp ) ...since you already have an F112 sub, I would think you have the bass part covered ( unless you just prefer full range speakers ) ...The m5/6's go down to around 60 hz or so with no problems
PS. if you get just the m5 or m6 monitors, you can buy the pedestals for them ( which are the b5 or b6 base pedestals without any actual woofers in them ) .. But imo to be honest, the pedestals are butt ugly. I just bought a nice pair of speakers stands for my m6's.
In the past I had the T5's and loved em... The 12 woofers were nice and tight and I really like the fact that I had a seperate volume control on the subwoofer section, so I could crank up the woofer now and again when I wanted to. The x1 also has some additional controls over the woofer. BUT, by itself wasnt enough as I still felt the need for a seperate sub for lfe duties. Now, in my present room which is kind small, I dont feel the need for having the B6 section for my M6's. ( especially since Ive aquired the JL F112 )