No one on this forum really knows about Sundown except Toby, who is a retailer for them.
Personally I would go with the Sundown SD1 over a W3, it is half the cost with probally close to the same output. Even though the W3 looks alot sexier lol. Other subs I would reccomend would be an RD Audio Studio 12, AA Assissan, maybe an AA Arsenal or RL-i 10 if you wanna spend a lil more money. But the Sundown sub/amp combo is going to probally be the best bang for your $ with very good SQ. Not to mention almost no drain on your electrical.
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MECP Certified.
Alarm: Viper 5200
HU: Stock
Active Set-Up
Tweets: Image Dyamics CD1pro mini horns
Mids: Sundown Neo Pro 8s
Amp: Kicker IX500.4 (2)
Sub Stage
Subs: (2) Dayton Audio 15" HF
Amp: Kicker IX1000.1