uneven depth of soundstage

My system consits of Quad 988 electrostatic speakers, Eastern Electric Minimax preamp, Resolution Audio Opus21 cd player, Quicksilver V4 monoblocks. The sound stage is reasonably deep in the center, but as the stage widens it quickly moves to the front, ending in the plane of the speaker at the extreme left and right. Almost triangular rather than an arc. Not on all cd's. What is leading cause of this anomoly?Pre amp, interconnects or just source material. Ive noticed this phenomenon more on audiophile cd's like from Maple Shade
Your comments would be greatly appreciated. I am a novice trying to understand this hobby
Your "good" CD's are more phase coherent, which in your case, is working against you -- because you have dipole speakers which are (I'm guessing) fairly close to the side walls? but pretty far out (> 3ft) from the rear walls? you may be getting cancellation around the outside edges of the speakers -- more pronounced with well-engineered recordings.

If I guessed right, then what you need is more absorption (drapes, blankets, absorbent panels) going about 6ft each way from the two back corners (behind the speakers) of the room.
The MiniMax inverts phase. You might try switching the positive and negative speaker terminals either at the speaker or the amp which ever is most convenient for you. Only switch at amp or speaker not both.
Nsgarch is on the right track but adding that much absorption will tend to make your room sound dead.

If you treat the corners of the room with Eighth Nerve products you will hear a major change. They make the corners disappear sonically, in effect making your room appear to be larger. They are coming out with a second generation of this product soon but the ones on the website work very well for not much money.

William7: Good call!! I would never have been able to make that great suggestion because I don't know much about that (Minimax) component. Carrecting the phase might be a very important fix, expecially when using dipole speakers!!. I'm impressed.
Nke, FWIW, set up is everthing with Quads and its not simple. It might be more helpful if you were to post the dimensions of your room, the speaker and listen chair locations, and a discription of the walls and furniture on the side walls and the walls behind the speakers. In the even you elect not to, I will just reinforce Hermans comments, you do not want to deaden the walls (side or back) near the speakers, you want diffusive materiels on these surfaces. Not to be grim, but it took me about 6 months to finally dial in my Quad 63's. With all the good advise here it may not take you as long, I hope.