Rich, The Eidolons continue to be the best speaker I have ever heard, but I've not heard the Kharma's, Sound Lab A-1/U-1 or some other contenders. If Jtinn is not using a fair amount of hypberbole in his comparison -- you should buy the Kharma (but which model is not clear). Jtinn has always seemed to present very fair and balanced information in this forum.
Nonetheless, for my listening preference (acoustic music, primarily classical), the Eidolons are superb speakers. They match the speed, clarity and coherence of electrostactic speakers and they present a holographic soundstage that must be heard to be believed. They are a reasonably easy load to drive, but they will be extremely revealing of everything in the signal path.
To your specific question comparing your current Opus to the Eidolon, Robert Harley wrote an excellent review of the Opus for TAS (Issue 122, Feb/Mar 2000) and made a very detailed comparison of the Opus to the Eidolon. At the time of the review, he had lived with Eidolons for a year and it served as his point of reference. You can download the review from the TAS website for a fee:http://www.avguide.com/product/Main_Speakers/Speakers/Avalon_Acoustics/Opus/545.jsp
Key points of Harley's comparison:
* Opus has a bit more relaxed sound, being a bit less forward in the upper midrange. It also is described as more forgiving of source and amplification shortcomings.
* The Opus has a similar sound through midrange and treble (same drivers).
* Very different bass reproduction, with Eidolon being "exceptionally smooth and uncolored, with amuscularity and extension not even approached by previous Avalon loudspeakers." The Opus has more midbass coloration, but perhaps more impact that some listeners may prefer - "a better party speaker, the Eidolon a more accurate reference monitor."
Hope this helps. If you like the Opus and value its strengths, I think you will really enjoy the Eidolon. For me, there is a very signigicant and worthwhile improvement in sound reproduction that goes well beyond just a more bass extension.