Daveyf, yes, as I tried to say before, I only use the fixed DAC output from the iDecco, and not the pre-out or amp. Thanks for looking into that for me, though. I think the DAC portion if the unit is decent. Mine was the first unit sold in Japan (certified), and as such I got it at a special low price that made it very worthwhile. BTW, I notice zero jitter with the Toslink, and hear no difference via USB. I'll trust you and keep it hooked by USB for the rest of this test, though.
Off to the shop now to see whether there is any way to try something by way of another amp, a sub, and/or cables for a reasonable price....