Hgabert, I need to be careful here as I very recently became a dealer which included becoming a dealer for the Foundation Research passive and dedicated line conditioners.
I should also note that I've been using the Foundation Research line conditioners for almost 5 years now and wrote several reviews here on these products years before I became a dealer.
I could be very specific about the line conditioner you mentioned based on my personal experience at a customer's home (If you email me, I'll provide you the gentleman's email address so you can discuss with him.)
Rather I will simply reiterate my statement in my previous post where I stated, "proper line conditioning."
Your sonics did not improve so much because of the dedicated lines because, like everybody, your AC from the street is going to be just about as dirty as everyone else's AC.
The dedicated lines would no doubt offer your amp greater dynamics and may have helped tame some AC noise generated from inside your home by appliances and dimmers now on other circuits but they do nothing to treat the worst AC noise which comes from the pole outside.
So if the dedicated lines were not the reason for the improvement, then there is still another reason for the improvement you claim.
Don't get me wrong here as I'm not trying to sell you anything. Except perhaps the thought that not all line conditioners are created equally. Line conditioners, like any other component, will run the whole gamut.
There are line conditioners that induce sonic harm, there are those that rob an amplifier of needed current and thus strip away the dynamics, there are those that do little or nothing, there are those that provide any combination thereof, and then there are those that induce no sonic harm at all while providing tremendous musical benefit.
Foundation Research products are the best I know but they certainly not alone in this regard.
Regarding your question about the preamp, like I said in my previous post, my hunch is that your preamp is lacking in the detail department. That's why you hear greater detail when it is removed. At the same time, the preamp is probably also masking the higher noise-floor, digital hash, and lack of properly treated AC at the same time.
My only advise here is keep the dedicated lines as they definitely have their place in a system. But keep experimenting with different line conditioners. You'll know when you've found the right ones for your system.
-IMO
