GrowSurf

GrowSurf is a more general-purpose referral platform than SparkLoop — it wasn't built specifically for newsletters, so it's used across SaaS products and apps as much as publications, but the core mechanic of giving subscribers a unique link and rewarding them when it converts works the same way for a newsletter's list.

It's a reasonable option if you want more control over the reward structure and referral rules than a newsletter-specific tool offers, though it takes more setup work to configure, since you're adapting a general tool to a specific use case rather than using something built for it.