Looking at your pictures I see a KB signal isolator, what does that do? Also where did you get the stepper drive from? Was that original to the machine?
I had a terrible time getting the analog 0-10VDC output circuit in the C11 B.O.B. to control the KB DC speed control. The instructions were not specific & clear enough (to me) as to the "isolation" factor and I killed one KB and vaporized a track on the C11 over the period of time I tried to get it to work.
I finally read the KB manual (free .pdf available online) more thoroughly and it clearly shows that to implement the voltage following feature, you either have to know exactly what you're doing re: the isolation thing (I don't) or use their KBSI-240D Signal Isolator which magically & totally isolates the source control voltage from the mains voltage in the KB speed control. All I know is that before I picked up one on eBay and hooked it up per the instructions, all I could get were dazzling flashes, fuses popping or the motor running at berserk speeds, totally out of control! It works great now and gives me exact Mach control of spindle speed to within 5 or 10 rpm.
The 1996 SmartStep/3 stepper drive board is an extra one I bought (used) from the Denford folks in the U.K. as a backup. Last year I had a long-running job using my little modified Denford MicroMill (w/lathe mod) and was afraid the original board might die on me before I finished the 1100 part run. The rectifier bridge in the original board died early on in the project and although I was able to replace it cheap and keep the board running, I was afraid something else might fail so I bought a spare. The ORAC came along earlier this year and I decided to use the SmartStep on it to keep the budget low. It's only rated at 2A/40V but seems to work well with the Xylotex 2.8A stepper motors. They're underdriven but in early test runs seem to work fine for my needs. I'm old & not in a big rush when I make things.;)
Hopefully I can make enough money with it to pay for the stuff I've had to buy for it so far and move up to some more powerful drivers & steppers later. The toolholder plate and twin QCTP's are heavy and will need more power to shove around if I start yearning for more speed.
Keep the pics coming as you move forward with your Prolight. I'll bet your toolchanger is common with those used on other small lathes.... like an Emco C5 or 6 or possibly even the Denford ORAC like was an option on mine. If so, the Mach thing has been done with them and there should be threads on how to hook up and program them over on the Mach support forum. Some of those guys are scary smart!