Hi twoch,
I don't have any experience with Cardas, but if you're using gold-plated connectors (i.e. spades, XLR, etc.) it could be restricting resolution. Also, the tube preamp/amp may not be able to reveal the full resolution of the different DACs. Gold-plated connectors combined with fully tube equipment may make a very warm laid back sound -- this may not be revealing enough to show micro-detail differences in source equipment.
I'm not saying this is the case, but it is possible. I don't have any experience with your two DACs. However, if you are expecting a radical change in sound, you will probably not get one unless you choose a wildly different topology that shapes the sound differently (like tube vs solid-state, or Class A vs. non Class A).
In another thread I stated how an Oppo BDP-105 sounded very similar to a custom DAC I had. However, when placing high resolution cables in the system (solid-core OCC, rhodium plated furutech), we could definitely hear that the custom DAC was revealing sounds that the Oppo just could not produce.