which dac dack, scott nixon, or other DAC $600?

I use and would recommend the McCormack DAC-1. It is very musical and the sound is very balanced and pleasing. You should be able to buy two of them used for the $600.00.

I just placed an order for the new Ack! dAck! 2.0 which is $695 until this Tuesday. After that it will sell for $795. It'll be interesting to compare it to my long standing Kora Hermes. Older Ack! dAck!s are going for a little over $300 used.

On a broader note, I wouldn't confine your search to just the DAC. What you're experiencing could be a number of other things such as component synergy, room acoustics, the NAD you're using as a transport, or your cabling.

A few years ago I was in a similar situation to yours. I was using a Musical Fidelity E60 as my CD player. I added a Bel Canto DAC 1.1 in hopes of solving similar sonic problems. The DAC helped a bit, but didn't fix it the way I'd hoped. What did was four years of trial and error and a lot of equipment changes.

Sorry if this further confuses the situation.