Hmm...having "dull' jpegs typically means that they are in the aRGB color space. That can either come from a camera shooting jpegs in the aRGB color space or, if you shoot Raw, the Raw processor converts to a color space it is set to. You can change this in Lightroom, Adobe Camera Raw, Digital Photo Professional (DPP) or whatever software you use.
So, at this point, please fill in some details! First, were these taken as jpegs, or did you shoot Raw?
And then, please walk through each thing you've done with the images from the camera into Photoshop -- some little detail can be the "missing link"!
And, check in your camera menu for the Color Space setting. Make sure it is sRGB for the "safest" setting.
Then, in Photoshop, take a suspect jpeg and go to Edit/Color Settings. In that dialog, you will see what Photoshop is using as your "Working Color Space". To make images "safe" for the Web, you can change that to sRGB unless you have reason to use another color space.
Check the "Warning" options in the dialog. You will need to decide how to handle images that aren't in the sRGB color space -- warn you but open them anyway, warn and convert them, etc.
Then when you are done with that dialog, close it and then open the Edit/Convert to Profile dialog.
Here, you will see what color space the image is in, and you will have the ability to Convert it to a proper profile (for Web use, sRGB). You should do this routinely with any images that are not already in sRGB for whatever reason. Or, in newer versions of Photoshop, the Save for Web dialog gives you the option of an automatic conversion.
I don't know anything about the PNG file format, so can't give any direct help there. But I am happy with the output from Photoshop, and haven't seen Photoshop doing a "saturation number" like you are describing.
So, there is another possible issue here, and that is the possibility that your monitor profile, which Photoshop uses, is having issues. If you have calibrated your monitor, you might want to do that again. If you haven't, well, you may want to investigate by playing a bit with your built-in profiles. But, since I don't use a Mac, I can't go beyond that!
Then, with the image open,