I would download the A3300SX operator manual from HiFi Engine-it's going to have the same suggestions for Bias.
You can certainly use the monitoring feature to judge which setting sounds right for your tape. Go for the best match to the source.
On your deck-setting 1 is for standard tapes and setting 2 if for higher bias tapes.
The "1"setting would apply to older tape types. These might include Scotch 150, Irish, old BASF, and many other budget tapes of the time.
The "2" setting would apply to somewhat later tapes, like the Maxell UD series (but not XLII series), TDK LX, Scotch 207.
If you have some tapes made in the 60's and before, then they are likely type 1. Type 1 tapes continued to be sold as budget tapes well into the 70's, but were eventually replaced by Type 2 tapes.
Keep in mind, I am using Type 1 and Type 2 in reference to your deck only-not to be confused with these terms when applied to Cassette tapes!
Welcome to the R2R world-it can be a lot of fun.