Next best exponential DAC quality level?

You need to define it for yourself.

Hint: it's not the DAC.

The cost for achieving state of the art sound, however, is high for both (at this point in time).

I will repeat using different words. In a clear, concise, easily understood, and repeatable, i.e. I can easily say something is state of the art or not, what defines a digital playback system that is "state of the art".

@martinman:

"I think the best way to sum up this thread is ...  you can get a lot of DAC for very little $$$". 

I'd suggest that depends upon what you like and more to the point, what you've heard.

A 2K DAC may sound incredible compared to a $300 DAC but less so when  compared to a $10K DAC. 

I have heard $500 DACs that arguably sounded much better than $10K DACs, and I have heard $10K DACS that sound virtually the same as $1500-2000 DACs.  It does not take a lot of money to make a DAC that properly recreates an analog waveform. Above a relatively small amount of money, it is purely specmanship and not audible differences.  However, if you choose to make a DAC that does not accurately recreate the waveform, something that a lot of expensive DACs intentionally do, then there really is no price/performance curve as it is all personal preference. However, human nature is to assign a higher quality to something purely on price.