Most (but not all) of the sample rate conversion algorithms I've listened to tend to brighten and harden the sound, anywhere from just a little bit to quite appreciably. The most transparent sample rate conversion I've yet heard is marketed by iZotope (Alexey Lukin's "64-bit SRC"). It can be had for a reasonable price in Audiofile Engineering's wonderful batch processor Sample Manager.
Might be worth trying a demo to see what you think.
Hope this helps.