I wouldn't bother with another sub unless its a real upgrade to what you have now. At the point you are at, you should be chasing after quality, not quantity.
Mixers do not have to cost $400 unless you want a lot of features. Definitely don't blow $400 on something that will just combine a few things. How many signals do you want to mix?
I wouldn't bother with another turntable unless you plan to do dance mixes, in which case you really should go for another Technics, or one of those DJ CD players that act like record players.
I wouldn't bother with a set of powered monitors unless you are unhappy with what you have now. Also, as with the subwoofer, I wouldn't get anything which is just 'so-so' or merely adequate. For your next monitor, get something that kills, like Adam, Genelec, high end LSR, Mackie HR, etc.
As for the tablet, I can't really say anything about. I just can't get into gadgets.
To sum it up, don't just buy something to spend money. For audio hardware, save your money until you can get something that is truly nice. I would also ask, how caught up are you on your blu ray collection? Lotta good sales going on and coming up, black friday, cyber money, some great discounts on blu rays. $400 would get you a nice big haul if you do your homework and get serious with price comparison.