Channel frequency range not balanced

True, following my previous attempts to locate the problem I firstly suspected the speaker then later ruled that solution out, although that was months ago. At that stage one of my tweeters had blown so I had disconnected the other.
On the advent of upgrading my speakers for the first time in thirty years, I again thought to source the problem.  Untilyesterday I was convinced that the speakers were balanced.
As for the improvement in the soundstage after reconnecting the cartridge I have no idea, I will play a couple of sides again tonight. I also realaigned and serviced my turntable during the tests, which I believe is important every few months. 

No disrespect but I think you should call your dealer over to have a look and make sure all is okay. 

If you think it may be the speaker, and you blew a tweeter it may be the resistor in the tweeter signal path. When you mentioned not balanced that is a trait of two resistors not matched if one is damaged. If one of the resistor is not producing the correct ohms you would experience what you described one speaker louder than the other.