Picked up another headamp: a Meier-Audio Corda Arietta. Big name, small amp. The amp is designed to run low-Z cans and I happen to have a pair of AD900X, along my Beyers that tend to present a much higher load to the amplifier.The Arietta is fairly new to me; I've just had ~6 hours of listening last night and it'll take a little while to see what it's really like. First impressions: dead quiet (quieter than the 105, which has no audible noise). So quiet that notes seem to emanate from total blackness. I realize I'm being a bit hyperbolic. Let's just say it's really, really quiet. The overall tonal balance is very similar with the Arietta, though lacking a bit of Oppo's air on top. The bass on the Oppo also seems to have a touch more weight, but it's hardly a night and day difference. The resolution - my personal holy grail - is almost the same, perhaps a touch higher on the Arietta.
This is the first real contender to Oppo's internal amp that I've heard. I'm truly excited about it. Will update the thread....