Been saving up for a big purchase. What to upgrade next?

Here is my current system:
-Soundsmith Aida Cartridge
-VPI Prime Scout Turntable
-Sutherland 20/20 Phono Preamp
-IN100se Integrated Amp
-Magnepan 1.7i

Room dimensions/Treatment:
-3.24m by 2.05m by 9.00m (W H L)
-112 square feet of diy acoustic absorption 
-16 square foot quadratic diffuser on back wall 

Listening habits:
-50% classical, 30% rock, 20% jazz
-Relatively loud level with average of 85dba with peaks in the high 90's

My current concerns:
-Lack of bottom end extension: Magnepans only go down to 40Hz
-Detail: I hear a decent amount of detail but I still feel like I'm missing something
-Imaging: The system is very lively but sometimes the performance sounds 2D
-Tonality: Fairly good but it can sound midrange is somewhat dry with an occasionally shrill top end

I'm thinking either a pair of subs, a new cartridge, or perhaps something else entirely. My budget is roughly $6000. 

If you guys are curious about what my room looks like I can try and post a photo!