I hate threads like these. I've ran both DD and Fi. I am a DD dealer.
The DD3500 is an amazing sub for the cost. The guys at DD are extremely helpful and if you go to any shows, you will see them supporting their customers in the lanes.
The FI BL is also a good sub, but has only a 1000rms (1500 might would be safe on it). I know people that run 3k daily to their 3500s
You know nothing about retail. DD subs are actually CHEAP compared to most any other brand that is sold in a car audio shop. We compare DDs $300 sub to most other retail brand's $600 subs. It is unfair to say Fi has a better price to performance ratio because Fi is only sold in one place and also can lower prices because they don't have any dealers to screw over. I'm not saying this is a bad thing, just something that shouldn't be compared to another company that has a completely different business strategy.
Any sub can get loud. It is all about install.