The M75 is not necessarily a dull cartridge (I guess it's what you mean by "flat"), it all depends on what stylus you give it and how you load it. Now if you want a really lively cartridge that works well on your 1214, you can try a Stanton 500. Cheap, and very enjoyable :thmbsp:
There is a cheap & dirty way to get rid of the 100 Hz rumble on those turntables, and that is to use it in mono. That's right, just sum the two channels, either electronically if your amplifier has a "mono" knob (preferred) or by wiring the cartridge in mono. The 100 Hz hum is out of phase so it will be almost completely cancelled out.