OH, right, forgot. I was having this same issue a couple of years ago and talked to Mr. Shram via e-mail. He gave me the impression that the DAC built into the P5 was just a utility, and that the DAC in the integrated was much better. I ended up going with a P7 with an external DAC.
The P5 is really a basic performer. If that's what you want for the price, it will do just that. However you mentioned you were looking for improvement, so I wouldn't recommend it to be all that for you.
If all you need is a DAC with volume, the Parasound VDAC V.2 is a better sounding package. Around $400 right now, but it has no analog inputs.
For around $1,000 new (Direct only), the Mytek Stereo 192 DAC has 1 set of analog inputs and a very good DAC with DSD capabilities.
There are probably other more capable DAC/Pre's out there you should look at as well.
Best,
Erik