The DAC I use that has two PCM63P-ks is an EAD CD-1000 Series III. It doubles as a CD/HDCD transport and DAC.
The PCM63P-k are 20-bit R-2R ladder DACs, the predecessor to the PCM1704-UK that is used in Audio-GD's flagship DACs.
The Theta DAC I also own is a Theta DS Pro Progeny version A. This is a PCM67P-k which uses an 18-bit/1-bit hybrid R-2R/bitstream DAC. It combines the topology of bitstream with a stereo 18-bit R-2R DAC. However, it uses a MotorolaDSP56001 to implement a custom filter algorithm (the same Mike Moffatt filter that was updated and handed down to the Schiit line of multi-bit DACs)
I haven't done an in-depth comparison of the two DACs using the same headphones and same amplifier. But based on over 6 months of owning both Theta and EAD, I would say they are remarkably similar in sound quality with the EAD getting a slight advantage in it's lack of sibilance or edge. The Theta, being a hybrid R-2R has a very extended top end, and is slightly more bright than the EAD. These are very subtle tonal differences. They can only be told apart when sitting side-by-side these two DACs.
Another difference between these two DACs is the implementation of a digital filter. The EAD CD-1000 series III uses a PMD100 HDCD decoder/filter chip to implement the filter algorithm. This filter algorithm is not custom, but was regarded well as being decent and used in many high end DACs from the 90s. With HDCDs, the EAD CD-1000 really really shines. I think it sounds slightly more spacious than when playing stock CDs and gains some of the spatial cues and ambience that is found in the Theta DAC with the DSP56001 w/ its time-domain optimized filter.
With standard redbook CDs, the EAD CD-1000 doesn't have quite the same spatial presentation as the Theta DAC. Definitely not a squashed soundstage, but slightly more forward.
Best way to sum it all up, EAD CD-1000 sounds tonally very neutral, without the overemphasis on detail like many sigma-delta DACs. Another piece of information, the CD-1000 was compared to the Lynx Hilo DAC which is a $2800 sigma-delta DAC that uses a Sharc DSP for digital filter settings. What we found as a group, was that the EAD CD-1000 has more lush midrange, less emphasis on the highs. It sounded slightly less detailed than the Lynx Hilo, but we found it more enjoyable and natural sounding in presentation. The Lynx Hilo had slightly better spacial cues and comparable bass, but not quite as good of a soundstage.(as the Theta, not EAD)
The Theta overall, was slightly more detailed with emphasis on the highs that was bordering the edge of a typical sigma-delta design but without sacrificing the drive and oomph in the midrange or bass. This DAC is like drinking a cup of coffee, it will wake you up and hold your attention tight. The soundstage is on-par with the Theta Gen V-A (Theta's TOTL DAC) but the difference is that its top end has a very slight amount of grain to it. Compared to the Bifrost multi-bit this grain can be slightly noticeable. However, nothing about the highs ever gives me the impression that it lacks detail or the difference in decay speed is that much different than the MB. The reason why I find it to be a better and more enjoyable DAC than the Bifrost MB is because of it's incredible dynamics and visceral bass and midrange. This DAC also has a 4V output, which can affect the synergy between DAC and amp... even when volume matched though, this type of sound has been compared to the Yggdrasil in a more recent Headfmania blog.
Link to DS Pro Basic review :
http://headmania.org/2015/06/28/theta-dspro-basic-oldie-but-goldie-dac/
Link to comparison of Yggy and DS Pro Basic:
http://headmania.org/2015/08/18/schiit-yggdrasil-dac-review/
Last thing to note. The DS Pro Basic uses 2x AD1865 converters I believe which are 18-bit versus the Progeny's 1-bit/18-bit hybrid DAC. The filter implementation is nearly the same, except the "version A" denotes that the Progeny was upgraded around circa 2000/2001 to use dither in the DSP and added 52 matched transistors to the silicon board.
