Yeah the problem with all vintage amps is that because they are old the sound can vary dramatically between the same models depending on the condition inside. Dying capacitors can really dull the sound compared to an amp that hasn't had much use for instance. Always clean the pots and switches with contact cleaner as there will be a build-up of oxides if it hasn't been serviced in a long while, this can make a dramatic difference too as the signal travels through all pots and switches.
Just a note on 70's Marantz in general and IME- They look lovely :) But there are much better sounding amps for a lot less money. Don't get me wrong, they sound nice and will power the HD800's well and add body - but what you pay for a Marantz you could have A LOT better sound (and a higher end model) for your money with a 70's Pioneer, Sansui, Trio, Yamaha or my particular fave with the HD800 an early 80's NAD.
This is obviously just my two cents as some are very happy with the old Marantz sound :)
Just a note on 120ohm, I would advise anyone who thinks the HD800 can sound a little dry or thin (and there are peeps out there! ;) to try a high impedance output and see if it pleases them. I think it can work great if you want a more bassy HD800. These days I tend to think it can be a little bit overkill as the sub bass isn't as tight as lower impedance amps... But its all give and take and what you lose you gain etc... The NAD amps I mentioned above have very low impedance value for vintage and they sound more 'right' to me.