in march of 2013 I got REW set-up and measured my <900ft^3 mancave with 2 klipsch Sub 10's and having some placement options didn't solve what I found to have no response <36-38Hz.
Since then I have built 4 subs, 3 of the Bill Fitzmaurice THTLP, a 24" and 2 30's, and , maybe you've read about it in the DIY thread, the SubMaximus, all FLH's
A good mod to make up for "how bad is it?" could be to add some shakers to the seating.
I put 2 of the $100 Clarks (209?) with a BASH 300 on my 3 seater couch and in conjunction with "Max" ,the XBOX1 rules, the usual, Titanfall. the new Wolfenstein, and especially that thermo scene at the beginning of LOTR. massive and violent and a great enhancement to LFE and any ULF you can get . .
I have 4 PE Auras on the loveseat in the man cave with a DAyton 100 watt amp . . ., not even close
get the Clarks, and you'll stop thinking about how bad it might be and
start thinking about how good it can be . . .
THE BIG ROOM: Mitsy DLP 73837 Xbox1 PS3 slim Panny BD 220
Denon X4000 and Emotiva UPA7 for 9.2
Klipsch F3 FL/R, G-28 C, RB 51's, FW's , RS3II SBL/R, RC3II, SL/R, SLX-FH's
2 30" BF THTLP'S and SubMaximus and Inuke6000DSP