Friend of mine had a pair of Bose 501s back in the mid 1970s, along with a US-made Altec Lansing SS receiver. Overall they sounded pretty good, but they really needed fairly high power levels to sound their best, along with "generous" use of the Altec's bass and treble controls . Probably not very suitable for most SE or lower powered tube amps, except for background nusic levels. .
If the cabs are in good condition, they could always be "repopulated" with more efficient speakers.
As I recall the 501 had a 10 inch (?) woofer, and one paper cone tweeter facing forward, and three "angled" tweeters firing out the upper rear of the cab. That precluded placing them against, or close to, a wall.
Unlike the Bose 901s, the 501s did not require the use of an equalizer.
I recently had one of the 901 equalizers that I bought for a buck at a flea market. It worked, but had absolutely no relevance to anything I owned, and when used with "real" speakers, the sound was awful
I didn't even bother trying t give it away, it was one of the first things to make a nice loud "clang", as I tossed it from the garage door into the dumpster in the ten yard dumpster I had when I was cleaning out the NH house, prior to my move south.
/ed B