Hrmm.... I don't know if I remember ever hearing the 501s. The grandparents-in-law had a pair of 601s that did a pretty amazing job of filling a large room with perfectly acceptable sound. The 501s look like a super big brother to the 301s.
No comment on the sound but with Bose speakers of that vintage, I'd definitely look carefully at the surrounds and even then, I'd assume they'd go to hell after having power applied to them for a couple days or weeks. A lot of time, they will look perfect, but perhaps they've been sitting, unused for 15 years. When they start moving, they might quite literally fall apart.
Not trying to scare you away and I'm not a Bose hater and truthfully, I might be slightly intrigued if I had a den type setting where I wanted a large stereo image over fidelity and detail, but I'd absolutely go into the purchase assuming I'd have to re-surround them within a month.