Tuning Rel G1

I just bought REL G1 Sub-woofer, which I connected with Krell S1500 Power Amp. I am using High Level Input with Synergistic Research Tungsten cable to connect. Speakers used are B&W 802D's. I went through REL manual 4 or 5 times, and followed the directions to set it right.
Problem is I tried crossover frequency from 32HZ to 40HZ, but I cannot have a volume opened more than 6 or 7. I spoke to several people and there recommended volume level is 14 or 15. If I set the volume level on sub-woofer to 14 or 15 my apartment shakes and base is extremely high.

Any advise?
Are you trying to add to the bass the B&W's can do plus pick up what they can't? I have my Rel B3 set to ONLY pick up what my speakers can't do. A lot of recordings just doesn't have deep bass. If you try to make up bass that isn't there you screw up everything when there is a lot of it. Remember the crossover in the Rel (or any for that matter) don't cut off where you set it just like speakers. The way you have it set at 35hz you are adding bass to what the B&W's already do. Humor me and set the crossover at 25 and tell me how it sounds. Then slowly cross it over higher if needed.

The other thing that would help is make sure the bass from the sub isn't leading or lagging the speakers. You may have to move it an inch or 2 at a time to find the right blend.
I totally agree with your recommendations. I did bring it down to 30 HZ but not to 25 HZ and the reasons are as follows,
I have a Home theater 7.1 system which is as follows

B & W 802D Front Speakers
Frequency range -6dB at 27Hz and 33kHz
Frequency response 34Hz – 28kHz ±3dB on reference axis

B & W HTM2 D
Frequency range -6dB at 35Hz and 33kHz
Frequency response 41Hz - 28kHz ±3dB on reference axis

Side Speakers
Monitor Audio PMC 702 Reference
Frequency response 35HZ - 20KHz

Mirage MX
Frequency Response (Hz, +/-3dB) 110 Hz - 20 kHz (+/- 3dB)

Now on my processor, I cannot lower down crossover frequency below 40Hz.
Just clarify, front speakers are set to full range for base and not "Full + Sub". I have a separate connection from processor to .1/LFE input.
Now front speakers have a frequency response from 34 Hz and up and that's why I set it to 35 HZ.
My questions are
1) If I set my sub-woofer to 25 HZ, than what happens to frequency from 25HZ to 35Hz which cannot be played by these speakers.
2) Several people commented that by adding this sub-woofer they got a wider sound stage. If I drop the crossover frequency to 25 HZ, how someone can have a wider sound stage, I am not very knowledgeable in this field but I think when someone says wider sound stage that means some of the mid-range frequencies are played through the sub-woofer. But will they at 25db?
3) For surround sound (7.1), if my processor has a lowest crossover frequency response of 40HZ and I set my sub-woofer to anywhere below 40Hz, what happens to the difference?

All these questions are because I paid $4500 for the sub and another $2500 for two cables one High level input and one for .1/LFE, so I want to do it right and I don't know “what is” and “who to” make it right.
Trcns First I'm not into HT. But I can answer your first 2 questions.
1) The Sub peaks out at 25hz but is not cut off completely. Like the speakers are not cut off at 34hz they are still going to 27hz but at a lower loudness. And the sub is still adding above 25hz again at a lower loudness. You want to find the crossover point where the 2 blend together.

That brings up phasing/timing. If the sub is 180 degrees out of phase with the mains you will get very deep bass but a huge suck out at the lowest the mains can do because the low frequency waves are cancelling each other. Don't rely on the sub phase adjustment to get them to be in sync. You may need to move the sub a bit closer or farther to achieve that. Again you don't want the bass from the sub either leading or lagging the bass from the mains. IMO that's the hardest adjustment.

2) Yes it will but minor. I didn't believe that until I got a demo with Dyn C4's and a Rel B1 playing a track of Diana Krall. Her voice sounded much richer and fuller with the sub. What was interesting it wasn't that way with all tracks. When I asked why it wasn't the same with all tracks he told me he couldn't explain it. But when he was trained by Rel to be a dealer they suggested that track to use as a selling point (BTW I don't remember the track or cd) .

Hope that helps