Hsu VTF3 or VTF15H... Outlaw x-13... SVS PB or PC 4000... Monolith 15...
Or something larger still...
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Best advice I heard and will pass on to you, here:
Buy the best single sub that you can allow yourself to afford now, then start saving for a second matching sub.
I lived for years with an undersized and underpowered sub. Never again. Yes it gave, um,
bass. But it wasn't until I had my X-13s that I understood how much distortion was involved, and how little output it gave. Now I have
BASS. Infrasonic frequencies and low distortion are at my fingertips. The cabinets don't rattle anymore... the house itself trembles!
If you are going to do it, do it right... or do it again later on, likely with a tear in your eye 'cos you will have already spent some money for inadequate woofage.