Hi Milpai, first I want to know which NOS Mazda's you are using with your Backert? I've got a Rythm 1.3 and have some tubes that Andy sent me which I like but love to tinker sometimes.
Next, I want to recommend the Maerason DAC1. In my rig (McGary/Backert/Fleetwood Devilles) it was a clear winner over Bricasti, Ideon and anything Chord. I know they have just updated the DAC1 to a MKII version and raised the price to $8K (rumor has it they want to introduce a new model in between the DAC1 and Frerot), but you can probably get a really good deal on a used version of the original DAC1. If you have a chance to hear one, it's worth the effort.