Pop-ups are generally pretty obtrusive to the user experience and may actually have a negative impact on your conversion rates. I'd recommend refining and testing your calls-to-action (including offer, copy, image, & placement) to make sure those are as optimized as possible.

I'd also recommend setting up your calls-to-action as "smart content" -- in other words, show different CTAs to your subscribers, leads, and customers so you're asking each visitor to take the next step that's most appropriate for them. Happy to discuss leveraging smart content in more detail if you're interested -- it's helped us tremendously in increasing our blog conversion rates.