I have all those boxes you mention but the JVC M50.
The JVC M70 will cost you the most ($200 - $500) but is the best built. Sound is very good and will withstand any type of music. Great "rock box". Has slots for a carry strap. Fun to play with and fantastic looks with the VU meters. JVC sliders go bad easily though.
The Panasonic 5085 is a very clear sounding box and should run you $75 to $180 for a nice fully working one. Sounds great and you don't want to push it past 3/4 volume but it's so loud you won't want to anyway. Again a great "rock box" and a real looker.
The Crown 950 is a new one for me. Suprised at the quality and I have seen these sell for $100 up to $350. Better bass retention than the Panasonic 5085 but not as good as the JVC M70. Good looking classic box. Crown did a great job on this one. Very cool LED VU meters. Not a lot of features to play with but it does a great job.
The Realistic 14-778 SCR-8 is a very cool box with the GIANT dual LED VU meters. That is a hard feature to pass up on. You could be looking at $75 to $150 for this one. The cassette decks are known to be kind of troublesome but when they work, they sound fantastic but good luck finding one with a working deck. Two built in hooks on top for a carry strap. Lots of buttons and a real "eye candy" kind of box. This thing sounds best under 3/4 volume and needs the bass turned down to get some real volume from it. Sound quality is right up there with the Panasonic 5085.