Snow Leopard does not recognize my 6MP using parallel cable

Probably both. Those adapter cables are for people who don't know any better and will buy them in hopes of saving a few bucks. About 99.9% of the time, you end up out the money for the cable AND a new device. Either the driver isn't set up to look for a printer with that kind of connection, or the cable is just flat out crap, it doesn't really make a lot of difference in the end.

So to bottom line it for you. You can either beat your head against the wall for a few days, hoping to get lucky and fix this, which you most likely won't... Or you can just accept that it's time to move on from your ancient printer and get something a touch more modern. I know the old LaserJets were built like tanks, and could wear out the energizer bunny, but odds are it needs a new fuser, transfer roller, and plenty of other things. The quality degradation has probably been so slow and subtle, you never even noticed.

You can get plenty of decent laser printers for quite cheap that will work with Mac OS X. Printers that won't cause the lights to dim for a three block radius when you turn it on. Samsung has a couple in their ML series that may be worth looking at in the roughly $150US range. Their PPM is probably a bit higher too. They do use the host CPU to do a lot of the heavy lifting, but a Mac Pro can shrug that off like it's nothing.

I have an NEC Superscript 870 Laser printer with a parallel connector that I have connected to my iMac running Snow Leopard with a Tripp-Lite USB to IEEE 1284 Parallel Printer Adapter. I works well. You can get it from Provantage, an online store that I recommend. The cost is between $25 and $30 with shipping.

http://www.provantage.com/tripp-lite-u206-006-r~7TRPA00T.htm

Originally, the NEC printer was connected to a Dell computer running Windows 98. The driver on the iMac from Gutenprint does not have all the features of the original NEC supplied Windows 98 driver, but it has the essential ones.