Transmission line spks:Close or far from back wall

I own a pair of Acoustic Zen Adagios which is a transmission line design with an oval port at the bottom of the front of each speaker. Over the 5 years of ownership, I have got a mixed bag of results placing the speakers in an average size room of 12X14 with 8--9 ft ceiling. But, I have encountered again another sound problem as a consequence of placement

I have had them placed 6 ft apart and about 40 inches from the back wall. This often yielded some good sonic results, but I was still generally dissatisfied with the overall performance. A recurrent problem is a somewhat bright over the top sound which can be hollow with reduced definition. Bass is very good. However, to my regret I made some changes which has brought on a new episode of mediocre sound.

1)About two months ago. I changed speaker cables to a brand called Grover Hoffman. There was (almost out of the box) a noticeable improvement in clarity, and naturalness of tone.

2) I replaced a Shunyata Diamondback PC cord with a Panagea AC-14SE supposedly designed for auxiliary compoents like CD players. The Shunyata had a junky plastic IEC female plug that oftem came loose during playback. I noticed a greater sense of loudness with Panagea PC used with the CD player.
3) I had my Ayre CX7e player brought to full upgrade status which is "mp" version. This upgrade was recommended by many members who owned Ayre CX-7e's. The end result was subtle but notable in terms of low end detail and overall coherency. The upgrade seemed to provide also a extended sense of loudlness which I attributed to a somewhat lowered noise floor.

4) I thought the overall sound of the system was good to very good despite few anomalies of loudness and tinges of brightness. Let's cut to the chase.

5)Speakers and components were placed at end of the long axis fronted by a large window, and a glass and wood door that leads to a small outside balcony. On both sides of each speaker toward each corner, a 7 ft bookcase was placed with an equal number of books records and mostly CD's. The right shelf near the balcony has been moved across the room, and down the outside wall placed next to another upright bookcase. Therefore, the right speaker wall has only a world map for sound reinforcement, and an overstuffed recliner several feet from the right speaker which looks forward. I cannot say forsure, but the last change seemd to make me more aware of the speakers new loudness, amd a somewhat over bearing top end, that is, a polar response that was somewhat above the back wall behind the speakers.

6) I decided to buy and use an IsoTek Enhancement disc over last week end. The program ran much quicker. and was quieter in execution of its process. However, yesterday was like Black Sunday, everything I played on the system sounded hard especially piano keys, unmusical and dead with little engagement.

For the record, the listening space I have is 12.5ft X15 feet. TThere is a medium size couch that functions as a room devider between the kitchen/dining area, and the so-called living room. Starting from the couch back to the front door, the room is approx.15X15 surrounded by kitchen cabinets and a 6 ft island counter behind the sofa which I lie to throw out the window.

In retrospect, I don't know what happened to the generally smooth, uncongested and detailed sound that came from the system PRIOR TO SOME, BUT NOT ALL OF THE ABOVE CHANGES.

I have been involved in audio for 41 years, but I am stumped, and at an impasse. Unless, I can improve the sound I have so I can listen contentedly. I have concluded that the system, especially the speakers will be up for sale. I am sure members will agree that a high end system's function is to thoroughly enjoy the music it produces, not wring your hands trying to figure its foibles and faults