Affordable is subjective. What I find affordable, you might find cheap or too expensive.
As I suggested the KRK 10″ sub is the logical addition to your KRK 6s. You can find slightly cheaper 10″ subs, but none will be tuned to your currenct speakers and, echoing what has been said in these halls many times before “you get what you pay for” in PA and speakers in general. So, not a good idea to skimp on the purchase of a cheapish sub. Better endure the lesser bass for a little longer til you have saved up enough to complement your KRK set with it’s matching sub.
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