let's first establish that brightness is the perception of excess sound pressure level in the upper midrange/lower treble.
it may be the result of subtraction in the upper bass/lower mids or a peak somewhere in the range 1000--3000 hz.
ok, what could be responsible for this. ?
i assume you did not notice this with the vt 100.
it is possible that there is an impedance mismatch between amp and pre. it could be that the ls25 has problems. it could be that the pass has problems. or, as you say, there may be something else about the pass/ ls25 combo that is creating the problem.
i would try other preamps, to see if the problem persists.
as has been suggested, there may be problems with other components, which the pass, being less "colored" than the audio research is revealing.
my experience with solid state is that there is also a speaker amp relationship which needs to be considered.
does the rockport have a metallic tweeter and where is it crossed over from midrange to tweeter ?
why did you replace the audio research with the pass ???
it may be the result of subtraction in the upper bass/lower mids or a peak somewhere in the range 1000--3000 hz.
ok, what could be responsible for this. ?
i assume you did not notice this with the vt 100.
it is possible that there is an impedance mismatch between amp and pre. it could be that the ls25 has problems. it could be that the pass has problems. or, as you say, there may be something else about the pass/ ls25 combo that is creating the problem.
i would try other preamps, to see if the problem persists.
as has been suggested, there may be problems with other components, which the pass, being less "colored" than the audio research is revealing.
my experience with solid state is that there is also a speaker amp relationship which needs to be considered.
does the rockport have a metallic tweeter and where is it crossed over from midrange to tweeter ?
why did you replace the audio research with the pass ???