Cecil,
I'm not sure if I can be of any help, but I'll give it a shot. I have a D&M 5 CNC lathe, which has a main control board, individual stepper drivers, and a separate motor control board. I too bypassed the control board and wired directly from a breakout board to the stepper drivers, which seems to be working. If your unit happens to be set up in this way, then the motor control board probably uses a DC 0-10V input signal to control spindle speed. I haven't done it yet, but I plan on controlling speed using a nifty board sold by CNC4PC that converts step and direction signals into DC 0-10V. It requires 12V DC, which I'm using an old laptop power supply to provide.
I had another thought on controlling spindle speed, but i have no idea if it would work or not. I'd like to try it, but I'd like to get my machine back together and get some workbench space back even more. Mach3 outputs a PWM signal that can be used to control spindle speed, and I had thought about running this 0-5 V PWM signal through an inductor to get a DC signal, then running this through an op-amp to boost it to 10V. I haven't ever tried using EMC, so this may not be an option for you, if it even works. Good Luck.
Adam