Maybe someone else here has and could elaborate.
edwyun, you bring up a good point. If the repositioning and the cable changes were concurrent, then there is convolution as to the contribution of the cables and the repositioning. However, if they were separate events, then the characteristics of each would be obvious.
This is one of the best questions I have encountered here in a long time. I have always considered the positioning of the speakers independently of the cables I use, however, there is no necessity of that. If a person wished to reconsider their positioning of speakers due to cable change, they certainly can, and may discover a preferable positioning.
I can envision certain circumstances where cable changes could potentially introduce the question of speaker positioning. i.e. some cables carry such strong low end emphasis that if speakers were placed near a boundary, or were in a highly reflective environment such as glass wall, then repositioning might be encouraged. It may also be potentially necessary to remove some treatment, the reverse effect due to cabling changes.
Imo most of the critical aspects of performance due to positioning are not significantly enhanced or diminished due to cable changes. Cables do influence dynamics, tonality and definition, but cannot resolve problems due to poor placement or vastly improve good placement. Once you move your speakers you have lost your reference. Not that it's a sin; do whatever you wish in terms of placement with new cables! Explore! Experiment! That's half the fun!
About a year ago I decided to revisit entirely my positioning of speakers. I moved the subwoofers fore, pushed back the mains, basically reconsidered everything and the result is wonderful! I should have experimented more a long time ago. Thankfully I was not forever bound by convention, and now have a much better experience.
So, I say have at it! If you feel the urge to reposition them, do it! No one can tell you that you are going about it wrong. :)
It seems most likely that something has changed in your settings in your player or DAC. For example are you using phantom to recreate a centre channel? For example, how is the oppo or cable box set up to handle downmixing of 5.1 TV signals? Is the player or cable box set to music hall or jazz or rock or Dolby Cinema or another of the multitudes of DSP settings (they all sound slightly different)
Check these settings carefully and toggle them and see if you can find out what has changed.
To clarify. The speakers were positioned a week or two before swapping the power cords mentioned (the previous ones are still around if I want to go back, which is not yet out of the question). It took two hours and a good wad of cash for two of them plus me to find the right placement of the ATC’s, and the techs were surprised and very happy, as was I, at how well it turned out given the odd layout/design of my living room and the placement of a 48" TV and components on a stand between the speakers. So you might understand that I’m hesitant to play around too much. At the same time, I thought it worth asking given the change in spatial characteristics and frequency curve brought by the new cords (e.g., the midrange is so far a touch farther back, although it has moved forward quite a bit over the course of burn in).
This is a 2.0 TV/audio system, with the Oppo set for stereo and the signal fed to the Direcstream via coax. Basically, I’m using the Oppo as a control unit and media player for the rest of the system. In setting up the Oppo, I turned off the DSP, DTS and any other effects and there's no reason they should have changed. There is no center channel. The speakers are almost a 1.5 meter XLR cable length out from the center of the TV. While there is not much more slack to extend, that never arose as a limiting factor in the speaker positioning; i.e., the speakers are where they should have been for the room and primary listening position.