Complex speaker placement is one of the most difficult things in setting up a home theater.
I think your problem might be four fold.
Problem #1 your GR60's are rated down to 28hz and if you are running them full range, they may be supplementing the bass output of the sub a causing standing waves of converging and diverging sound pressures, depending on the cutoff frequency. Cure - decrease the subs cutoff frequency or raise the main systems cutoff or both to see if anything happens. Always invest in a sound meter. It will be the best $40-50 purchase you ever made.
Problem #2 Corner loading a sub is the best way to augment SPL levels due to reflection and horn loading, however, when dealing with multiple sources of bass output, it becomes much more complex. This is why you find when placing the sub close to the main speakers, the sound improved because as you increase the central point source of the bass output, the sound overlap of the waves becomes much tighter (from my understanding) with less holes in the standing waves. Think of it as dropping water into a basin. A drop of water at two points 1 inch from each other simultaneously will yield a tight pattern of ripples. Trying this again with the drop sources on either end of the basin makes for a tremendous amount of converging and diverging areas. "Sweet Spot" location is very difficult with this setup with three sources of bass. With a dual subwoofer location of either side of the room both corner loaded is the loudest way to make bass as there is room symmetry but once again you have to move them about or play with phases to make them work for your "sweet spot" area. This works well with satellites with limited range or with the fronts set to small with most of the bass redirected to the sub. You may want to try this as well.
Problem #3 : Your room dynamics may not be conducive for good corner loading. Without symmetry, or a good "flat" wall to increase reflections, placing the sub in front of cabinetry with multiple angles and multiple places for sound absorption/dispersal may not be ideal even though it might be corner loaded. Surfaces that surround it might also be a problem such as high shag carpet, textured wall surfaces, lots of drapery that is always down, proximity to couches or behind couches with thick foam or soft surfaces, multiple paintings on the wall for sound dispersal, pitched roof, vaulted ceilings, the proximity to open doorways/ recessed windows, etc. There are many variables. If this is the case find the longest flattest surface to place the sub upon, possibly behind you if necessary. If you place the sub directly in front of those accousic panels I think is a defeated purpose if they are rather thick. Most panels work for higher frequencies and allow the lower ones to reflect. I don't know which you have.
Problem #4 - Its time to play with the phase control. This is the hardest of them all an requires time to set, a good test cd/dvd and patience and a sound meter. A small twist and listen and back and forth and back and forth.
cbraver - I love the crawling on the floor thing...too funny
Effective but funny.
Tips: If you are having difficulty, try to move your front speakers forward and back to change the location of the standing waves, same with the sub. Move your sweet spot "couch or seating" location a few feet around back/forth. Place some sound absorbing foam on the surfaces behind the main speakers if possible to catch some of those reflected waves. Your acoustical panels might also be causing the problems on your side walls, not allowing the bass to propagate into the room. Panels should be placed where you do not want sound to reflect to increase point source and increase immaging from my understanding but will rob you of reflected bass sound (which corner loading is trying to increase) if they are too thick. Try removing them and see what happens.
What probably might make the most difference is to have the sub built into the cabinetry in the front in a central location. I might be wrong, but I think that will help things greatly as the sound will be enhanced by the vibration of the wood, being flush with the cabinet for more even dispersal and having a hard surface to reflect off of effectively.
A complex speaker problem is what cedia technicians are for
If all else fails, time for them to make a house call.
Hope that helped somewhat.
