I purchased a CNC kit off eBay that had TB6600HG Drivers. I wasted days trying to fault find the intermittent issues and finally came to the conclusion after reading forums that it was an issue with the drivers and that they had a design flaw.
I found steppersonline and bought 4 of these digital units, wired them up, and my CNC was instantly 1000% better. One of the huge differences is how quiet it makes my Nema24 steppers. They are so quiet that you can actually hear the bearings on the rails not the motor.
The other benefit is the resolution. The TB6600HG do 1/16 at best (if they are working), these do 1/256.
The extra investment is far and away worth it for the time saved and improvement.
I bought the 7A units as I am upgrading my CNC (1200mm X 3000mm) to NEMA 34.