Hey David, prerecorded tapes, just like records, will have varying amounts of tape hiss depending on 1) the master tape it was recorded from 2) the equipment it was recorded on 3) the type of tape it is recorded onto. Depending on the era the original recording was made (and other factors such as tape speed and width) the master could be the origin of the loudest hiss. Tape formulas and production techniques have come a long way as has the record and playback equipment used on the master tape. Some of my older tapes were manufactured in the late 50's-early 60's. If you consider that the first US magnetic tape machines were made as upgrades of the Magnetophon recorders confiscated from the Nazi's at the end of WW2 it's a wonder that they even existed that early.
I don't think that the fault is with your 1520. It's the tapes themselves. The signal to noise ratio should be constant. Another thing that may help a tad, is there is a selector for tape eq. Try the different settings. For me, the earlier tapes were made to a pretty high standard. They may have more tape hiss, but they generally sound better.
Regarding your other questions, we all started at some point so don't worry about asking questions. There is however, a sticky posted at the top of the General Forum. In it you'll find tons of important info regarding tapes and machines. If you check out the US Recording Media link there (under the information heading) it will take you to their "tips and faq's" page. It's really excellent and required reading if you value your tapes. Of course, they want you to buy all your supplies there too and since they're one of the only places for some of this stuff, I highly recommend them.
