brwsaw, i feel you.. and know where your coming from... This is part of the reason i got 2 600's, with a 3rd "in the works". You could do like john said and turn up the subs volume.. Also, you could adjust your cross over points, my 600's are set to 80hz i think.....
The only real way you can get what your looking for is by having large drivers, like you mentioned. as well as multiple drivers...
Maybe a single EP800, if you can't get dual 600's?
What is your current sub? post a link so we can see where you're coming from.
If Ian made an 18" driver sub, i would buy one... Just saying... Incase Ian or andrew come across this thread......
