Our experience matches ICUToo's. Our REL Gibraltars placed on the Isoacostic SubDudes need about 2db more gain in our room.
Another option to try is to spike the subs. We feel proper spiking is the best solution sonically providing room coupling is not too severe. Spiking does provide significant structural decoupling and also provides better bass articulation and tonality than the SubDudes, but the SubDudes simply can't be beat for easily and dramatically decoupling the subs from the room.
There is however a slight penalty in articulation and tonality with the use of SubDudes. Not so with spiking.
We have installed spikes and discs on our beautiful 3/4" hardwood floors in our showroom on both our Gibraltars.
If you have hardwood:
Solen here in Canada sells VERY nice sets of four with self tapping inserts for less than 50 dollars a set of 4.
Dayton Audio DSS4-BK Black Speaker Spike Set 4 Pcs.
solen.ca
Simply drill four 5/16" pilot holes ~1/2 deep into the base of your sub, install the inserts so they're flush with the bottom of the sub thread the spikes and your done.
If you have light carpet, commercial carpet, or carpet tiles:
We reccommend the following set:
Dayton Audio DSS2-BC Black Chrome Speaker Spike Set 4 Pcs.
solen.ca
Installation method is the same as for the hardwood set.
For medium to deep carpet we reccommend:
Dayton Audio DSS3-BK Black Speaker Spike Set 4 Pcs.
solen.ca
GOOD LUCK! And remember, the best thing you can spend on your system is time.
Kerry