The digital volume controls on the squeezebox work by changing the bits. In other words, there is some digital re-sculpting of the signal, and it's lossy.
Supposedly, it gets worse as you attenuate more and more, and I don't know at what level it is "still OK".
I don't know if the Transporter does the same thing, but I would be surprised if it didn't.
Hence the standard recommendation to let some other device do the volume attenuation. Yes, those are lossy too--but in an analog way, and that's usually better by comparison.
Before passing judgement on the DACs, you could insert a preamp--any preamp, probably!--between the DACs and the amp and then do some a/b testing. The difference you have been hearing between the two units may go away, or become a different difference; hopefully, in both cases, you will hear them play better.
Supposedly, it gets worse as you attenuate more and more, and I don't know at what level it is "still OK".
I don't know if the Transporter does the same thing, but I would be surprised if it didn't.
Hence the standard recommendation to let some other device do the volume attenuation. Yes, those are lossy too--but in an analog way, and that's usually better by comparison.
Before passing judgement on the DACs, you could insert a preamp--any preamp, probably!--between the DACs and the amp and then do some a/b testing. The difference you have been hearing between the two units may go away, or become a different difference; hopefully, in both cases, you will hear them play better.