John,
In my system and most of the systems I have listened to here in Colorado, I found the best bang for the buck to be with the digital inline purifier.
Now, most of these system were using the ART DI/O modified to MENSA level, as the DAC of choice. You may have a different opinion with a different DAC. I have also found the purifier to make a big improvement on the digital audio signal of my DVD player into the digital input on my Lexicon Pre/pro.
The next biggest bang comes form using the Bybees in your AC line.
The Bybee Power Bar or inline AC purifiers reduce the whole system's noise level.
Once those two are done, then adding them to the speakers make a big difference.
I have also found adding Bybees to the input of a phono stage to make as big a difference as the digital line.
Please realize that each system is different and each person's idea of a big improvement is different as well.
If you look at the posts on the Bybee inline purifier thread on the BOLDER Cable circle, you will find overwhelming amounts of positive comments about these devices.