Well, I'm not sure about other cards, but with the Hauppage units, it does indeed affect the capture. I literally see it with my own eyes when I load the clips into
VirtualDub for editing. The Hauppauge proc amp is limited though, with only Brightness, Contrast, Hue, Saturation and Sharpness available to be tweaked. I've successfully brought down the brightness and saturation, adjusted the contrast, then leaving sharpness as low as it will go.
And although it does reset on its own on my WinTVHVR-1850... sometimes, that usually doesn't happen until reopening the app of choice when capturing, and not unless a reboot of the pc. At least not in my experience.
The USB-Live2 is a different beast though... after figuring your settings then selecting "apply", as soon as you select "ok"... it reverts back to the default settings again. There's more than a few comments out there of people having the same issue, including a thread here on this site. The workaround in that thread was to use GraphEdit.
https://www.digitalfaq.com/forum/video-capture/8511-usb-live2-proc.html
I would have posted my findings in that thread, but it was too old. Just trying to help anyone else who's facing the problem.
Cheers.