I agree that speakers have the best goody/$ ratio. My wife won't go with treatments (though I have done what I can and pull the drapes closed for critical listening).
I'm pretty happy with the passive speakers I have. My best speaker is the Focal Solo6 Be which is an active studio monitor. It has a 150W rms, BASH amp to drive the mid-bass and a 100W rms, class AB amp to drive the tweeter.
Here is a link for the curious:
http://www.focalprofessional.com/en/sm6-line/solo6-be.php
It is quicker (more articulate) than the passive speakers I have, but also has more power (I presume) so I was wondering if the a bigger amp might close some of the gap. In the end, it is probably the active configuration which really makes the Solo's different. I'd especially welcome any comments on this!
I would certainly entertain any ideas on speaker upgrades, especially active speakers.
But it is pretty difficult to get higher end speakers in a true A-B scenario.