Fernando - i completely agree with your preamp suggestion as my good friend has lent me a Gryphon LE preamp for a whole year. with the Gryphon LE, i've heard a more vibrant, vivid, and effortless sound from my system. it didn't change the signature of my system, but rather improved it many ways, and more importantly, music was more enjoyable with it. but i wouldn't say the preamp elevated my system to another level. given the very high cost of my dream preamps - Jadis JP80 and JPS2 - i believe the biggest bang for the buck improvement for me is to get better speakers.
on the subwoofer - i think best placement would be dependent on the end goal. seamless integration of the sub is more important to me than getting the lowest possible frequency response. therefore i choose to place it closer to my monitors. as fantastic as the Velodyne room correction software is, i wanted to use the equalizer minimally with signal purity in mind. i am thrilled with the results. if the sub shake my windows a bit more they might just crack!
one thing i haven't tried is elevating the sub by 20 inches per an audio dealer's suggestion, which may help with integration. just need to find a sturdy stand that is good looking as well...
thanks for sharing!