Paul R is completely correct.
The TDA1541 is usually partnered with the SAA7220 digital filter chip.
Unfortunately the SAA7220 is a noisy power hungry beast of thing and puts quite a lot of 'noise pollution' into the surrounding circuitry. It also handles the clock signal distribution too and unfortunately degrades the quality of the clock signal in the process.
Removing the SAA7220 (and thereby removing the oversampling and filtering) will leave a load of ultrasonic noise/distortion in the signal that shouldn't be there and would usually be filtered out. This obviously seems like bad idea and backward step.
However.....removing the SAA7220 also removes the noise, power supply and clock signal problems associated with that chip so there are some benefits and some people prefer the resultant sound which is often described as being 'warmer' or 'more organic'.
A much more optimal approach though is to give the SAA7220 it's own dedicated and well sorted power supply and rework the clock distribution scheme so that SAA7220 is no longer handling the clock signal distribution duties. Once you've done all this you'll be getting all the positive benefits of the oversampling and filtering and will have removed the problems caused by the SAA7220. However doing all this is quite a lot of work and the costs add up.
The NOS conversion by comparison is quick, easy and cheap which largely explains it's popularity.
I've tried NOS conversions a couple of times and it's definitely NOT to my taste.
Incidentally, Sony based machines using the TDA1541 use Sony's own digital filter chips which seems to have fewer downsides than the Philips SAA7220.
