I wouldn’t call the volume issue a defect since rotary encoders are known for sometimes skipping a step, going two steps, or flipping back one on occasion. It’s a byproduct of using an analog signal to control a digital parameter. Encoders immune to this would be prohibitively more expensive. The fact that some of us may get a “better” unit has to do with the tolerances of the component.
That said, the soldering on the production units isn’t the best I’ve seen and a lot of it is done by hand. It’s not a bad idea to open the unit up, inspect the board and clean off residual flux and splatter, and put some thermal interface on the heat sinks. A lot of small defects can be spotted by eye and resolved easily.
intermittent headphone channe could be as simple as one of the socketed op amps not being fully inserted, or even having some flux residue on the pins. The assembly is one place topping needs to improve their products. Unfortunately, it’s an expensive upgrade.
Both my toppings had some mess from assembly and the first one had a dead headphone channel. Turns out the right balanced channel was shorted to the left so the left worked but the right was phase cancelled between two channels. I cleared the short and the unit was perfect. Topping sent me a new jack free of charge too.
Topping has the design and components right, they just have a few more inches to go on QC and assembly. That’s unfortunately the hardest part to iron out on a budget, but if you’re handy enough you can do a little clean up in your own.
