Is it nessasary to have a USB converter w/ NAD M51



IS IT NECESSARY TO HAVE A USB CONVERTER WITH A NAD M51 DAC?

I'm using my Sony XA-5400ES as a transport for most CD listening. I use a Toshiba computer directly into the Nad 51's DAC USB port
It sounds good but would a converter (like a Stello U3 or something like it) make a difference in the sound? Would it sound better or just different?
Has anyone done a comparison with and without a USB converter with a NAD M51?
Thanks for reading, all answers are considered helpful.
128x128zeal
Not necessary, but in my case using a Musical Fidelity VLink 192 did improve the sound a bit- smoother and quieter, by a small margin. I found the same to be true for my Eastern Electric Minidac plus.