Good to know that you have sealed the source side. Did you mod the 999ES with multi-$k stuff?
And I am glad CSP2 does the justice for you high-Z phones. My understanding of the input level adjustment was to cope with DACs with 2.5V or higher output level and also already high-gain of CSP2. The change of tonality is kind of side effect (but pleasant one). Steve did mention that by lowering the input level the music sounds more "open" and not "compressed" and I can relate his comment with my listening experience with CSP2+.
Skylab does mention high freq roll off with LCD-2. Was that your problem? No matter what, there might be more reasons to use SS and transformer-coupled amp for low-Z phone than just gain setting, such as current vs voltage need.
LOL Source is no problem and you might be surprised since I do own the CSP-2 at how good the SS Matrix is and performs. My T1 and 770 are great with either amp but I do prefer the SS with the D7000. The new amp supposedly has volt adj to adjust for low impedance cans in my conversation with Steve Deckert