Unless you can do it yourself, $700 is not enough budget for good bass in that size room. 3000 cu ft isn't huge, but it's big enough to warrant some proper subs.
Just above $700 gets you the best bass for the buck sub; a HSU VTF-3 Mk5.
I'd still recommend a pair, but if you've got a $700 budget right now, save some pennies and get the best sub you can get in that price range (VTF-3 Mk5), and then start saving for it's twin.
Multiple subs have numerous benefits. More output, much less distortion, more even bass response at multiple seating locations... More truly is better if you set them up properly (and that means external EQ in 99.9% of cases with something like a MiniDSP 2x4HD, REW, and a Umik-1).
MiniDSP 2x4HD, UMIK-1, REW... The holy trinity for running multiple subs.
Espon 5050UB, 120" screen, AppleTV4k, Harmony Elite, Sony UBP-X700, Pioneer Elite VSX-LX504, Outlaw 7000x, MiniDSP 2x4HD, 2x HSU TN1220HO subs, 2x HSU VTF-15H Mk2 subs, Defintive Technology speakers: CLR2002 x3 for FR, C, FL; BP2x SR & SL; BP1.2x SBL & SBR; ProMonitor 800 x4 height speakers.