First thing I would do is get a gage. Factory turnouts are notorious for out of gage locations in critical places. If you look on the Internet, you will find a number of articles on how to improve them. Then you have the DCC freindly issues.
Remember these parts will have variation, so some will be better than others.
I suspect the C-C truck loco is just more sensitive to the issues.
Your dash 8 might also have issues because of it length.
My feeling is that if the unit is supposed to run on 18 inch radius, it will. The desire for more realistic looking models has led to the sensitivity that is haunting you. Thus, the track work for 18 inch curves and number 4 turnouts has to be nearly flawless. Going to 22 inches and number 6 turnouts helps, but takes more room.
The other consideration is that most modern rolling stock models being made today are intended for 22 to 24 inches or higher. It is something to think about when you select what to buy. Kato has some recent Amtrack HO models that will run on 15 inch curves, so there are some things available. In the US, the prevailing thought is for large radius curves, etc. This is mainly because trains look toy like on sharp radius. This is true, but the larger radius takes more space. The interesting thing here is that geometry will show that you get more straight section with the sharpest radius. Clearly you can't have square corners, but are 24 inch or larger really required? Kato's customers want 15".
Back to the original post. There is no reason why a no sound DCC version will be noisier than a sound version of the same release model. What is likely happening is something either came loose or was not where it was supposed to be in the first place.
These models are not run until you take it out of the box. They are put together some where in China, shipped in containers across the Pacific, worked through the docks on the west coast, and shipped to the importer and then to either the stores or the end customer.
I see a number of new engines in a year. Today, most are really well packed. At least half have parts floating in the package or inside the engine. Often the engine will not run. Either something has lodged where it binds the drive or a wire has come lose or broken.
I never send the unit back. Usually with 15 minutes effort the problem can be identified and solved, leaving a perfect unit.
I know, you spent close to $200 for a model that comes broken. The QC varies from one importer to another. However, the factories and path to you are nearly the same. Keep in mind that the odds of the replacement unit also having a problem are high. Likely a different issue, but a problem none the less. These are the realities of limited production fringe products like model trains.
Don't get me wrong, the models produced today are really fantastic. But they also have more opportunities to have flaws. The buyer today is less tolerant of these flaws for many reasons. To keep from going in circles, you sometimes have to fix the flaws yourself.
Your last choice of an Atlas unit was your best selection, I find them to be one of the importers that has fewer issues, but the do/will have some.
Good luck,
Larry
www,llxlocomotives.com
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