The sound is lower because you lowered from 10 to 5 dB. Its fine.
The gain knob on the subwoofer should not be at 12 o'clock but rather at the position labelled "+0 dB". Sub settings left to right:
- Crossover maximum setting
- Phase 0 Deg.
- Level +0dB (from there, adjust to taste on your receiver)
- Crossover switch THX
- EQ THX
- Power as you want. Auto On makes the most sense.
You then need to go back to page 2 of this thread and read 3dBs posts where he shows pages from the manual to your receiver. That shows in which modes the subwoofer actually outputs sound. You obviously need to use those modes for the sub to work, and you need to be aware that the receiver may store settings per mode, which means the receiver may change your subwoofer level for example differently for Dolby ProLogic and MultiCH.
You speaker settings/layout in the receiver should be 2.1 (assuming you have FL, FR and subwoofer only), and front speaker size set to small. Cross over is a matter of taste, but try 80 Hz in the menu on your receiver for a good compromise.
Those setting should definitely give you a good starting point, unless there are other issues at play.
From there, you should be able to play material and hear/feel the sub. If you still do not, please be accurate in your description of the issue, as the thread is a bit confusing to read (at one point the sub shook the room, now its umimpressive as I gather). For example, if you do hear/feel bass from the sub, but would like more, try increasing the level in the receiver. Does that help or not? If not, try increasing the level on the back of the sub. Does that help or not? Etc.
It could be that there's something to the cable stuff, and it could be that your sub is in a really poor position in the room and thus has it's output "sucked out". But it's very difficult to discern without accurate feedback from you, and mostly I think it's a "trap" made of the combination of your experience and a very confusing receiver. For example, it makes very little sense that it will simply not output to sub in certain modes, and it should always show a layout diagram on the display to make it clear what it is doing, not needing to review tables in the manual.