Been thinking for a while now about connect a vinyl deck, or two, to the SQ5, for sampling. Has anyone got any ideas of the best way to do it and what extra equipment might be required 
Never going to do it, but curious if anyone has done a DJ mix off an SQ or something similar.
Whatsoup,
A simple solution would be to connect the turntable to any XLR input by using a stereo DI box like the Radial AV2.
Then you could record a number of different ways, SQ Drive, Tracks Live, etc…
A DI will absolutely not work!
Any phonograph playback requires an RIAA preamp.
What I would suggest:
Turntable into 3rd party RIAA preamp
RIAA preamp into ST1 or ST2 (via RCA to 1/4” cables)
I would suggest using the ST1/2 inputs instead of the regular XLR inputs as you have no need for the mic preamp in this situation.
the radial j33 di has a riaa preamp integrated. i think that would be the most elegant way.
Budget alternative might be the iConnectivity SpinXLR.
If you went with the Audio Technica AT LP 120 USB turntables, they have line outputs. You could go straight into the two pairs of stereo jack inputs with that. Two sets of phono leads, which may come with the turntables, and four jack plug to phono socket converters will get you connected.