systems like the 33/303 - because they were designed for sources with really low level output compared with the 2-3vrms+ anything today can deliver - have rather higher 'gain' overall; also likely higher input impedance too, which itself, esp in conjunction with the higher source-impedance from a poorly-chosen value of passive attenuator, can be small added source of (Johnson) noise. possibly even hair of HF roll-off, with the wrong attenuator value, the wrong end of a long interconnect.
So - you take a modern source, attenuate it a lot, feed it into such a system where you wind up the vol pot thinking 'that looks about right' - you loose stuff into the noise floor, because you've heavily attenuated the signal, fed it into a system with a noisefloor not as pristine as is possible today, then used the available internal 'gain' to amplify-up to yor desired output. Just feeding the hotter raw signal in, and attenuating the output with the system vol pot, even at markedly lower settings, might, might well... sound better, overall.
There are a lot of 'ifs and buts' in the above - it depends on several aspects of whatever the older amp architecture is, and impedances involved etc - therefore also, quite a bit of maths or other direct measurement req'd if you wanted a more exact answer - but that is the essence of it: it depends...
Pick what makes you happy.