Input limited

You can use the Tape1  and Tape2 "PLAY" as a line level input.
For example, connect the CD player output to Tape1 "PLAY" input, switch on the Tape Monitor1 for CD playback, switch it off to select other source.

I think they will all work, you have a tape monitor too? That is fixed, everything else is selectable and line level except the phono section.
Isn't tape 5-600 ohms too?  I got to brush up.. I know there are different EQ curves. I'm just now getting back into it.. It's for recording and dubbing, (I think) Mic input, and head phone.

I think this is correct:

The inputs were named so front labeled designations for playing or recording tapes from/to Tape DECKS (often two Tape Decks) made sense.

Tape DECKS have internal equalization, then it’s line out, line in. So, you can use any line/tape in/out/loop/ to play any line level signal. Also any Tape DECK into any line input.

IF it says Tape HEAD, then it is only good for a tape signal that has not been equalized. The device would have internal tape equalization, similar to built-in phono equalization.

Therefore, Tape HEAD input cannot be used by any other device, line level or otherwise, just as a MM Phono input which has built-in phono equalization can not be used by any other line level device, or MC weak signal. 

Certain era equipment, prior to standardization, had options for various equalization curves for both Tape and LP’s, they had several built in equalization circuits, all fed from the Tape HEAD or Cartridge.

Sorry gents. I should have mentioned I have a Sony cassette deck and a Teac open reel. The tape in/outs are used. My SAE 5000 is using the Adapter loop (originally for a Dolby unit). My EQ unit will be in the pre in/out loop.

I am left with Aux for the CD and Squeezebox.

So you are out of inputs. Your options now are to use one RCA and switch it from CD to Squeeze, or 2 RCA and swap them at the amp. Or if one of your tape decks is seldom used for recording, then you could run the CD or Squeeze into the REC input of that deck, and when you want to listen to it flip the deck to REC and monitor the line level through the deck.