I owned an Audio Research SP-5 in 1980 for nearly ten years. For its time it sounded very good… it was the most expensive preamp I could afford (used). Great solid construction. A bit lean on the midrange and bass.
The way I look at it, there has been… let me guess, nine successor preamps (I have owned or heard many of them) and in each case the new one sounded significantly better than the former. So, that adds up to a lot of difference. ARC is a good example because they have had a very consistent target on achieving realistic high fidelity over the lifetime of the company, not wavering around on flashy or chase specifications… of exaggerating some aspect of the sound.
But of course it depends on what you are after. The best sound you can get or stopping at some threshold that makes you happy.