Vintage electronics gurus: would you kindly help me troubleshoot my turntable home audio set up?

I recently purchased a Kenwood KD-45 fully automatic turntable from Craigslist and a 70’s HH Scott 320R receiver. (Both were advertised by their sellers as being in perfect working order)

I set everything up and I was listening to records earlier today through cheap headphones with no issues at all. I turned everything off and went about my day. I came back from getting a huge haul of albums from my grandparents a few hours later and got ready to listen to a pristine copy of Meet the Beatles. To my dismay, the volume was very quiet, even when I turned the knob up, and there was a great deal of static, pops, and distortion to the sound. I tried the new LPs I was listening to earlier and they had the same issues. I tried a different set of headphones and had the same problem. Thinking it might be a headphone issue, I went out and bought some Koss over the ear headphones that a few people have recommended to me. I got home and plugged them in, turned everything on (with the Beatles album in again) and the sound was beautiful and clear. Just as the album went to the second track, the volume dropped dramatically and the distortion/crackling sound came back.

I don’t have any speakers, so I can’t really troubleshoot everything, but I’m thinking this may be a headphone jack issue. Can anyone here help me figure it out?