I want to use the Play Electric as a multi effects processor in front of my tube amp. Is it possible to use the equalizer without using the 'Guitar Amp' simulation?
No, this is not possible.
If you set the "Guitar Amp' control to off, you will bypass this section and the equalizer. To maintain your guitar tone and have access to the eq, choose the "Acoustic" style (not acoustic shape). Set the drive to -9 dB. The "Acoustic" style gives a flat response and no coloration in the sound. The drive set to -9 dB will give a unity gain between input and output, no gain boost and no gain reduction on the input level.
This will give you access to the eq without adding any extra amp modeling.