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!
I would look at a different amplifier with more current delivery. All Magnepans thrive on power, the more the better. Within your budget there's plenty of options, especially if previously used is acceptable.

I don't have personal experience with the 1.7i, but I've used various Maggies for over 20 years (currently the 20.7) and they all need a lot of juice to come alive. All your concerns will be resolved by using more high quality power. Even the bottom end will seem to go deeper. Adding subs to Maggies will do that too, but not necessarily to your linking. And it won't address the other concerns.

The simple solution that will get you truly awesome state of the art bass is a Distributed Bass Array. A DBA is simply four subs distributed asymmetrically around the room. The number of subs and having them spread around is far more important that which subs are used. So this method allows you to buy four moderately priced subs. Most of us have only about $3k in our complete DBA including the amp.

Here is mine.   This is just one way it can be done. The easiest way is to just buy the Audiokinesis Swarm system for about $3500, although it sounds like you might be in EU land, no idea about shipping. You can also simply buy any four powered subs. So that takes care of bass. 

Now the detail problem. I don't see any of your components being known for flat, dry or shrill sound. For sure it is not coming from your front end, which is superb. Don't mess with that! But I don't see wire mentioned at all. So the answer could be as simple as spending the rest of the $6k on good used Synergistic Research interconnects, speaker cables, and power cords.  

Never heard Maggies myself, but a lot love em and they are very well regarded in spite of as you noted not always being the easiest to drive. You can spend an awful lot of time and trouble trying to find the amp that can make difficult to drive speakers sing. Some enjoy doing this. Not me. I would rather stick with easy to drive speakers knowing it frees me up to find the very best amp without worrying will it drive my speakers. 

In that vein you might want to listen to this guy, who absolutely loves his Maggies but look at the way he talks about his Tekton Double Impacts.
He starts talking about his Maggies and Double Impacts at 4:45.
I own Magnepan 1.7i (and I love them.)  I've driven them with a class D amp, class A monoblocks, and a high-current class AB amp.  In my room, more current gives me more bass.

I do not use my two powered subwoofers for regular listening anymore.
Thanks for all the feedback guys! Seems like everyone is recommending better amplification. Do any one have any recommendations for a good power+pre combo? Bryston’s products look pretty nice.