my small room is also with in your dimensions, 1 door, 2 windows <900 ft^3,
I had 2 Klipsch sub 10's "smartly" located in the room
Then I ran REW and found my response at below 38 hz all but disappeared.
Short answer I built a THTLP 24" wide ,from Bill Fitzmaurice plans. using a BASH 300.
So have you considered DIY ?
for your budget you can get a really great ROI for varying degrees of effort
and some of the flat packs offered around here, just too easy to not say no, thank you.
keep us posted
Denon X5200 , Emotiva UPA7, for 7.3.4
Klipsch F3 FL/R, G-28 CenterLow, Icon 25 CenterHigh, RC3II: SL/R SLX: Rears ,TF & TR, RB51's:FH
2 30" BF THTLP'S and SubMaximus and Inuke6000DSP
Mitsy DLP 73738 Xbox1 PS4 Panny BD 220