Jeff, unfortunately the Grado RA-1 has no pass through RCAs (two sets) as the best place to put Grado is tape out of preamp, that way you can easily turn volume down on speakers when using headphone amp.
If you want to always have cassette deck connected I would get one of the various splitter RCA adaptors for tape out of preamp so cassette deck and Grado Ra-1 can both use tape out.
As Beemer stated you can turm amp off so no sound goes to speakers, but this may not be ideal as you may want to listen to speakers after headphones, and now amp needs to warm up. I would get RCA splitter.
If you want to always have cassette deck connected I would get one of the various splitter RCA adaptors for tape out of preamp so cassette deck and Grado Ra-1 can both use tape out.
As Beemer stated you can turm amp off so no sound goes to speakers, but this may not be ideal as you may want to listen to speakers after headphones, and now amp needs to warm up. I would get RCA splitter.