How much lower do you think you can go? You may have another issue besides just the brand of subwoofer if you're feeling like you're missing bass. Have you tried repositioning the sub? Where is your crossover set at? How big is your room? How low do you think your sub should go?

Try answering those questions before investing in new equipment. The bass may be there already but other factors are preventing you from hearing it. A little experimentation may produce some great results.