Good question Gary, since I forgot to mention that I use a Stello USB/Coax converter between the USB output of the Mach2 and the Havana DAC - I tried to use the Oppo directly to the Havana via the USB output but - to my ears - it sounded a tad better with the Coax output, not sure how the Havana is wired as for different sampling/convertion rates between its different input options. (It is fair to say that I used a regular USB cable in my test).
Would you suggest to go back and try with a decent USB cable directly from the Oppo to the Havana DAC? I am all ears!
The Stello converter is quite good, but not the best. It's USB powered - which means that any switched-mode noise will be transmitted. Galvanic isolation in the Stello is also not the best. You should try the Mach 2 with USB direct into the Havana.
The best USB cable is one which has the power pin removed. If you are careful, you can do this yourself. In my experience, cost/looks has absolutely nothing to do with how well a USB cable performs. Some USB cables have the shield connected to ground at the host, other USB cables have the shield connected to ground at both ends. Many USB cables have the shield un-connected to ground. Depending on how "dirty" the ground on the sending device is, the ground pin may have to be removed as well at the DAC end if that is connected to the shield.
http://pinouts.ru/Slots/USB_pinout.shtml
From the following picture, I don't see a pulse transformer at the USB input, so I doubt that the USB is galvanically isolated. This means that depending on the configuration of the USB cable, it will sound different. Try my favorite which is a white Belkin 2m USB printer cable (bought for $18 at Best Buy).
If you are in the mood for experimenting, I would also try the Toslink output from the Mach 2 to the Havana DAC using a good glass optical like the Sonicwave ($25):
http://www.amazon.com/Cables-To-Go-Sonicwave-Toslink/dp/B0009JR4GK/ref=dp_cp_ob_e_title_0