Typically when used in a stand-alone or master/slave mode, you're not going to get control over dimming. If there was something on the switches, it would be in the manual.
I suppose if you wanted to, you could put some gel sheets in front of the lights to knock it down a bit, say using some grey or even super thin black.
What you might want to see is if in the manual there is a sound active mode option in the DMX specs.. This would give you the "lack of control" that you've been enjoying. I'm not going to say anything negative about this lack of control, it's a good cheap way to get an "instant ligth show". But, you might also want to see if that while in an sound active mode you also have control over dimming.
You may be needing to get a DMX controller of some sorts to totally get what you need done. Just because the manual says it is an option, it may be a "conditional" thing, where that condition is that you have to be in DMX mode to be abl to get that option as something you have actual control over it.
Are you planning on using these live? If so, the brightness you're complaining about now would most likely work in your favor for your shows. I could easily see a bunch of guys crammed into a room with their gear and these lights just blowing you all out. But live, ideally, you SHOULD have significantly more space to spread out and do your thing.
Hey, I'm all for a set-and-forget quick and easy light show. At the same time, I want control. I can have both but it takes extra work on my part.
I suggest getting a DMX controller of some sort. If you choose properly, it should grow with you and provide the features you want and need now and at least for 3-4 years into the future. Typically, most people often outgrow their current controller for one of two main reasons: Too many lights and too few channels on the controller, or need more features than their existing controller offers.