What would you pair the DJM-T1 with?
Assuming price was not a factor.
I think the DJM-T1 and 2 X1’s or midifighters would be a great controller setup. The DJM-T1 seems more club friendly that the S4 with the outputs. Might still be a bit overkill though considering the S4 can do just the same as the DJM-T1
The DJM-T1 is quite different from the S4 and what would you need the 2 X1’s for? There is already one build into that 2 channel mixer that supplies the needed midi controls.
But to answer your question: Since Traktor Scratch Duo 2 is paired with it, i guess the best match would be turntables or cdjs. Everything else feels like a waste, because there are better mixers for these combinations.
2 brains to then make me realize I wasted money
By the looks of it extra X1’s shouldn’t really be neccesarry. Perhaps one for sample deck control (although you probably can fix that with the midi-buttons on the mixer.
Definetly a pair of TT’s. What a clean setup that would be…
2 technics…
2 2000’s 
I would’nt even buy one even if I had the money.
Would buy the DB4 instead.
This.
To answer the question though turntables.
Id prefare the xone 4d
X1’s would be for 1-8 hot cues and true control over 4 decks. All the above comments kind of echo my thoughts, but for some reason I still really want that mixer…
Might not be what your looking for but I just ordered the Livid Code and new OHMrgb for £730 and I’ll have more options than you can shake a stick at for less than the price of the S4!
the whole point of the thing is to use it with two turntables and timecode for a battle style setup… If you were going all midi this would not be the mixer you want.