which output setting sounds best?

Why don't "you" try listening to them to find out????

That will answer your question. Who else has your system or your ears???

^^^^ ditto ^^^^

I don't think it matters. You are not getting anywhere near a decent signal with the USB/HRT. It is all veiled anyway. Doubt you will hear a difference.

I have been using a Wyred4Sound Dac2 which is asynchronous. Also using J Rivers 16 with Vista. Wyred4 sound recommends Kernel Streaming, but I have switched numerous times back and forth between KS and Wasapi Event with almost no perceivable difference. I have noticed a bit of a shift in imaging from left to right with Wasapi Event seeming more centered.

Give them all a try. Its an easy experiment.

Windows Audio Session API (WASAPI) is the Windows-resident interface provided in Vista and later OS versions that supports exclusive bit-streaming to a specified device in native resolutions up to 24/96 via USB 2.0.

There are likey other issues with Vista 64 that may lead you to want an upgrade to Windows 7. But WASAPI should be resident and if so,

This is definitely the selection of choice - unless your DAC supports higher resolutions, e.g. 24/192 or otherwise directs you to another driver.

Media Center includes WASAPI as a standard option. Run WASAPI in Event Mode if possible (In some cases you may need to increase buffer size from the default of 50ms).
This allows your DAC to impose control of packet timing and size.

BTW, Media Center's core algorithms take full advantage of 64-bit mode when available.

Good Luck.