My YouTube channel and videos have Google Adsense enabled.
Can I ask viewers to "like/subscribe" on both ?
Would it violate the Google Adsense Terms of Service?
Violating the Adsense TOS would be asking people to click the ads in your videos. Asking people to like/subscribe is pretty much the same as asking people to bookmark your site and come back.
If you ask people to click on your ads, the advertisers lose money without getting the chance to generate income, so this is against the TOS. Asking people to come back will generate natural ad views, which is exactly what they want.
Yes, it's perfectly okay to ask your viewers to like & subscribe (assuming your content is original). Monetisation of your content doesn't make any difference.
For more information, read youtube's terms here: http://www.youtube.com/t/terms
Unfortunately AdSense program policies and YouTube’s Terms of Service clearly forbids manipulating users to click "Like" or "Favorite" because you try to get them repeatedly viewing your video in the future and potentially repeat clicking on Google Ads which are shown in your YouTube Video.
Here are some key policy violations to keep in mind, however make sure you read the policy thoroughly as well:
Manipulating or incentivizing others to click on video features such as “Like” or “Favorite”