IMO the Resolution Audio Opus 21 would be the best of that group with Ayre coming in second. I am also a believer in the GNS upgrade to the the Opus 21 but it is a bargain either stock or modified. The Wadia 302 is Ok but somewhat colder and more digial sounding.
You may not hear all the differences with the Marsh 2000b preamp though.
You may not hear all the differences with the Marsh 2000b preamp though.