Sub for sub almost never works and is a pretty bad way to advertise since you're not getting active engagement. You might be able to get people to click on your comic and inflate your subscriber count, but that doesn't really matter if their not engaged with the work itself. Think about it this way, would you be constantly keep up with every comic you subscribe to during a sub for sub? If 30 people do it, would you go out of your way to even care about properly checking out their comic?
That's why read for read or (more preferably) review for review is a lot better. Having someone sit down, go through your work and actually give feedback is invaluable. If they like the work, congrats you've gotten yourself an active subscriber. If they don't, congrats you've gotten yourself some constructive feedback.
I understand the desire to want to increase your numbers, but the more important way to look at it is how your numbers are increasing and what that number actually represents. I would take 50 active subscribers over 1000 inactive ones any day.