The motor "sings" if you PWM in the audible frequencies, ie 20hz to around 20khz. Most industrial servo amplifiers start at about 25khz. That's about as low as you want to go to avoid the motor singing. Except for being annoying and having courser current regulation, lower frequencies won't necessarily hurt the system. I'm not sure what your circuit is, but you should check out IGBTs for switching amplifiers. It should say right on the data sheet what the rated switching speed is. There is usually a grade right around 25khz, and another grade around 100khz. Another good spec to look at in the data sheet is the current rating at high temp, usually 110degC or so. It's dangerous to design around the 25degC current spec for something like that. It's going to get hot.
You didn't say what voltage you want to run at, but for 12A continuous you are probably going to want to use PWM. If you go linear you are going to have to have a huge heatsink.
A good place to start learning about servo amplifiers is the OSMC (open source motor controller) project. They use parallel TO-220s in the air, which you could replace with one TO-247 screwed down to a backplate, but still a decent design. Then once you get that you can start adding current feedback and make a real controller.
Careful with filtering the output of a PWM. The components all need to be rated at the current level of the system, and 12A inductors are bigger than you think they would be at frequency.