Caveat Emptor?

I was getting a haircut this week from the son of the barber who has cut my hair for the past 30 years. A couple of years ago, I sold the dad a Pioneer SX-3700 and a Thorens TD-145 (with a Grado cartridge) at a ridiculously low price. While cutting my hair, the son mentioned that he was getting into vinyl and was looking for a turntable. He knows very little about stereo equipment, but plays live music. I told him that I had four turntables and wasn't really looking to sell any of them at this time. When I got home, I did an inventory, and I actually have five tables (three Thorens, a Music Hall 5.1SE, and a JVC QL-5). Although it's the newest of the group, I felt I could part with the 5.1SE for the benefit of a budding audiophile. I called him and gave him the good news and of course, he asked "How much?". I told him he needed to come demo the table and see if he liked it, then we would talk price (I had the figure $400 in mind). The 5.1SE probably has 600 hours max of use time on it, and it is in like-new condition, plus I upgraded the RCA connection cables a couple of years ago (through a fellow A'goner). A couple of years ago, I picked up a very nice Realistic STA 120-B receiver off of CL for $150, even though I had no place to use it. It has been sitting on a spare shelf in a rack since I bought it for the most part. I intend to throw the 120-B in with the turntable, because he mentioned he was currently using something from Zenith. The 120-B needs dial lights and a tuner realignment, but everything else (including the phono section) on it works great. I set the pair up this afternoon and connected a pair of Aperion 422LR speakers to the 120-B, intending to give him a "worst case" demo of the gear. Once the receiver warmed up, the sound was surprisingly good, IMO. The 422s were sitting atop of a pair of Monitor Audio Silver 7s, and at the end of the record, I switched the speaker cables from the 422s to them. Holy cow! That was a whole new ballgame and I was ecstatic over the impression that would make on the young man. After that record side finished, I approached the set to turn it off and heard a very faint 60hz hum coming from the speakers. I had not heard it from the 422s, probably because of their very limited lower-end response. I rechecked the ground-screw connections on both the table and the receiver, and they were nice and tight. Of course, the hum level is volume level dependent, but I never heard it while music was playing. I pulled the original stock RCA cord that came with the 5.1SE and was surprised to see no grounding cable at either end of it. I did not bother to swap that out with the upgraded Supra cable, but will give both to the buyer for him to experiment with. I do not plan to mention the hum at the demo due to the fact that he may not even notice it. If there are any other 5.1SE owners out there, I'd appreciate any feedback you might have on any similar experience. I think it's the connection on the 120-B because I've used the 5.1SE in several systems and never had a hum problem at all.