Tim, I thought interpolation *was* "upsampling", whereas increasing word length was accomplished by adding randomly generated dither below the signal level, so it sounds to me like *you* might be confusing two separate things. But then again, what the heck do I really know about this stuff?! Not a lot, let me tell you!!
To address the main question, I just yesterday received a new-to-me MSB Gold Link upsampling DAC bought here on the 'Gon, to compare with my reference Theta DSPro Basic IIIa (Theta also seems to disagree with the theory). I've just started auditioning, and will post my findings when I've had enough time to make a meaningful decision about which unit I will keep. But I can tell you right now that the MSB with upsampling engaged definitely sounds different not only from the Theta, but from itself with the upsampling turned off, so it ain't doing nothing. Finding an upsampling-capable unit and auditioning it against your reference may well prove to be an educational experience - I think it will be for me.
To address the main question, I just yesterday received a new-to-me MSB Gold Link upsampling DAC bought here on the 'Gon, to compare with my reference Theta DSPro Basic IIIa (Theta also seems to disagree with the theory). I've just started auditioning, and will post my findings when I've had enough time to make a meaningful decision about which unit I will keep. But I can tell you right now that the MSB with upsampling engaged definitely sounds different not only from the Theta, but from itself with the upsampling turned off, so it ain't doing nothing. Finding an upsampling-capable unit and auditioning it against your reference may well prove to be an educational experience - I think it will be for me.