I have just finished auditioning a used California Audio Labs DAC with both my HT receiver (Kenwood 6060) and my two channel receiver (Onkyo 8011). This took a heck of alot longer than I had expected. My HT is housed in a LARGE entertainment unit so changing cables is a bit of work.
My main reason for trying an external DAC was my new facination with Internet Radio. I was pleased with the way it sounded with my two channel, which has one of these running a USB digital out to analog,

http://www.xitel.com/product_phfl.htm

but I knew the Ascend's in my HT were capable of more than I was getting running an optical cable direct to the Kenwood.
In my HT, where the Ascend's reside, I use the dac in my DVD/CD (Toshiba 290) player for two channel by running the analogs to the receiver. When comparing this set-up to the digital out from the Toshiba to the Cal Audio then analog to the receiver, I could not tell a definitive difference in sound. The Toshiba does a nice job and the Cal Audio did also.
When I ran the digital out from the Xitel to the Cal Audio, there was a small but noticeable difference in the sound from Internet Radio. It was my impression that the sound floor had dropped. Everything just seemed a bit clearer. No one aspect seemed to jump out. Just an overall improvement. This is just what I was looking for. Now the bad news.
The Kenwood has a red light that signals clipping. With the Cal Audio engaged, this light would periodcally flash briefly. This does not happen with the dac's in the Toshiba or the Kenwood. There was no clipping apparent when the light would flash nor was it dependent on the volume. The light would flash in the same place on the CD regardless of SPL. There are several possibilities of why this might occur but there is no inexpensive or less time consuming way to find out. I'm choosing to reject the DAC based on this anomaly.
I also tried the Cal Audio in my two channel system but did not hear any improvement over what I already had in place. This is a very nice low buget system (Mission 761 2-way speakers, Toshiba 2710 DVD/CD player with the Onkyo and Xitel).
If not for the clip warning light being activated, I'd have kept the DAC just based on the improvement in Internet Radio. If the Kenwood did not have this "feature," I'd have never known of a potential problem.
Onward and Upward.

David