I looked at the owner's manual and I see no form of tape monitor function. However, I would hesitate before taking the Onkyo back. I'm familiar with the 901's and it's eq. My mom has a set of 901's. I think the new reality is that these speakers and their special needs have had their day, although they may still be nice for stereo.
Consider this. The only connections to get proper surround sound from any source is through digital connections, whether HDMI, coaxial or optical (barring, of course the six channel analog). When using the 901's through a tape monitor, you are limited to using stereo analog connections through the tape monitor and you will not be able to use any digitally encoded surround sound format.
My suggestion is to keep the Onkyo and buy a normal 5.1 speaker setup for home theater and consign the Bose to a dedicated analog stereo setup elsewhere.
Or, return the Onkyo and buy an older analog receiver with a tape monitor from a pawn shop or yard sale, because as long as you're using the 901's, you have absolutely no use for any of the modern digital inputs that provide surround sound:
Unless you return the Onkyo and buy a receiver that has 7.1 preouts and additionally purchase a separate multi channel amplifier, then plug the Bose eq in between the FR and FL reciever output and FR and FL amplifier input.
Cliff Notes: time to replace the Bose.