Can upsamplers alter a CD's sonic performance?

Sal- Of course, that's what the manufacturer's claim. Regrettably, there are plenty of products that don't live up to their hype. Let your ears be the final judge of a products performance, not a spec sheet playing "numbers" games. Just my .02

Sal, I am only familiar with the Purcel and what it did, regarding soundstage, dissolving powers, presentation of voices and solo instruments and smoothing out that otherwise sometimes a bit brash and forward highend, I found quite amazing. I could not stand listening to redbook CDs without it anymore. Mind you, I'm basically still a vinyl man.
Happy listening!

I strongly believe that with the right DAC, upsampling improves the sound of a regular CD. I own both the DCS 972 upsampler with the Elgar DAC and the MSB Platinum upsampling DAC. I've also tried the Perpetual technologies in front of a Theta Pro Gen Va upgraded to accept a 96kz signal. All of these DACs also oversample in addition. The dCS web site has some very usefull and not particular self serving technical white papers on this subject.
To keep it simple; the reason that upsampling improves the sound is that it allows the DAC to convert the digital info to analog with fewer added artifacts that are a result of the DAC's internal conversion algorithims and filters. These added analog artifacts (which are NOT part of the original CD's encoded data) make the analog signal sound harsh and congested. That said, the best perfomance should be from a DAC that is very well designed in the first place. A fair number aren't, power supplies and the quality of the analog stages being a typical weakness. In dealer's systems, I've also listened to most, but not all of the contenders in the high end upsampling DAC market niche. There are subtle differences between the high end DACs that can be brought out by different recordings. Along with the standard "You need to let your ears be the judge" you also should listen to a representative sample of your favorite music. Because of this, it is hard to say which of these combos is the absolute best.