hi peter
thx for the response. must say everyone i've talked to speaks glowingly of the CAST system, which helps explain your results.
i'd guess w/ a diff pre/amp combo (one that isn't designed to work together), you might find the 581 more competitive w/ the ARC as it would preclude the use of a preamp and make comparisons one of ARC+pre vs wadia sans pre.
i'll be getting a 581 in soon enough myself and will see how it shapes up (my preamp is nigh on the best avail).
best
ryan