Speaker configuration Marantz 2270

I just received a Marantz MIJ 2270 from the late 70's. Looking for speaker opinions. I tested the unit on a pair of very cheap pair of bookshelf speakers. Granted the speakers were terrible, but I could tell the system could benefit from a sub; or a full range tower with bass speaker.

I am working within a budget of about 400-650ish. My experience with high end stereo equipment is limited, but am familiar with live speaker gear. I like the punch you can get from monitors, but have to keep in mind I would need a speaker that would like a lower powered signal being I'm working with 70 watts per channel, another reason to look at smaller
speakers.

In doing a search I noticed many people felt a sub of some sort is needed. I have found a few older passive subs with smaller drivers that had decent specs. I would like to entertain a passive sub as to get more the Marantz sound versus a powered sub. I would be concerned in blending
two set of speakers not knowing if I would need a crossover or not.

Being i am a newb I may be completely off base with my assumptions.
What suggestions could any of you provide me for my quest?

Thank you in advance