Upgraded banana plugs


I recently upgraded my standard gold plated banana plugs with Audioquest Suregrip bfa banana plugs, which are silver plated over copper, to get a better quality connection and to hopefully add a little extension at the frequency extremes. What I ended up with was a totally different sound. I went from a warm, musical sound to a much bigger and brighter, dynamic in your face type of sound. I lost a lot of the tube "magic" afterwards and it was like I replaced my tube amps with a solid state amp type of change in sound. Can anyone explain how changing just the banana plugs changed the sound so much? Is the gold/silver plating more responsible for the change or is it the better grip/design of the plug?

Here is my equipment lineup:

Tekton Lore 2.0 speakers
Cary Sixpac monobloc amplifiers
Audio Research LS15 preamp
Squeezebox Touch > PS Audio Digital Link iii dac
Speaker cables are Belden 89259 (Jon Risch recipe)
Interconnects are Belden 89259 terminated with Eichmann Bullet rca's
rkc5666
For another upgrade (which is what the new plugs are), replace the binding posts on your speakers and amp with Cardas Rhodium over Copper ones. Brass reeks.
Great responses so far. I never would have guessed something as basic as connectors and binding posts could make such a large impact on a systems sound. I feel humbled to say the least. 

Based on the advice thus far, before changing any components I should get the speaker/amp binding posts changed out and then see where my sound is at that time and work my way up from there. A popular upgrade for the Sixpacs are replacing the standard coupling caps with Jensen paper in oil caps that could bring back some tube magic as well.

Kacz, my Squeezebox does have a volume control so I may give your suggestion a try however I have subwoofer control and tv sound into the LS15. I could try nos tubes in this unit as well. Lots of options but I'll start with the basics first. 
May I suggest simply a trial of bare wire.  I had some higher end LAT International speaker cables with expensive locking banana plugs, one of which broke.  I stripped all of the bananas at both ends and re-connected, the sound opened up and improved in every way.
May I suggest simply a trial of bare wire. 
+1   The best connector is NO connector, if you can use it that way.