FWIW, I have modified a '90s-era Kenwood DP-1001 CD player with TDA1547 DAC,
and it pretty much leaves stock Marantz CD600x players behind in audible sonics.
They key mods are in the I/V stage, where I have an LF03d discrete Class-A
bipolar opamp in place of the stock monolithic JRC opamp - more info here:
LF03 discrete opamp - Page 3 - diyAudio
It's a direct replacement for an JRC4565 or similar, which is tricky only because
of space constraints and relatively low rail voltages for the I/V stage. I had previously
tried (among several others) the LME49722 and 2x OPA627 as the I/V, and
they were all disappointing (though mostly better than the JRC). Most good
monolithics choke on the low rail voltages, apparently, and those that thrive
on low rails (e.g. LM6152, LM6172, AD8022, etc.) aren't great as I/V converters
or are very fast and have stability issues in the Kenwood DP-1001 DAC board.
The LF03d gets around all these issues, because it's relatively slow (< 10 MHz
GBW), runs on rails as low as +/- 3V, and on audible sonics A-B comparisons
it pretty much leaves the stock CD6003 behind after the upgrade.
The Kenwood has very good build quality, and all mechanicals are fully
functional even after 20+ years. Highly recommended, if you can get
hold of one. The only tricky upgrade is the I/V opamp, but it was worth
the effort.
