How do I get the best soundstage?

I haven't listened to your speakers but they may not be the best for soundstage.  If I was in your shoes, I'd consider that.  Try to borrow some more conventional 3 way speakers with an exceptional midrange.  Midrange is where the soundstage exists.  Not sure how horns affect soundstage.  Some others with horn speakers may chime in but I would wonder if the horn design is affecting soundstage.  

Agree with speaker placement.

Also, for soundstage, I think your best results come with the minimalist approach.   The less times the signal is touched by the system the better your soundstage will be.  Pass is a good amp but ss amps always do a lot more to the signal.

Jerry

@hysteve , getting a system (room included) to image correctly is very difficult given the equipment most people have. Most of the electronics, cartridges and even speakers are capable of doing this at a reasonable even surprising level but the system needs to be calibrated to the room it is in and any other errors corrected. 

For a proper stereo image symmetry is the most important factor. The performance of both channels (room included) needs to be identical in phase and frequency response. Early reflections have to be absorbed or speakers need to be directional to prevent them entirely. It is not important that the frequency response overall be flat but if the treble is up 3 dB in one speaker it has to be 3 dB up in he other. The channels need to be matched as closely as possible to prevent smearing the image to one side or the other certainly within 1 dB 100Hz to 12 kHz.  Digital signal processing is the only way to achieve this. You can get within 5 dB with room treatment if you know what you are doing. To know what you are doing you need a measurement microphone. Digital room correction preamplifiers will do it for you. The Anthem STR is in your price range. DEQX is releasing new units in about three months. They look great but the prices are not yet available. Trinnov currently makes the best unit but it is $11,000. 

Some people will never experience a system with SOTA imaging because they are digital phobic. But, you seem to be fine with it. The only thing I would change in your system other than getting a Digital Room Control preamp is, get rid of the unipivot tonearm on the VPI. It is a terrible arm. Get an arm with proper bearings. VPI makes one now that is an easy exchange. 

Imagine if clubs and concert halls and all the other assorted auditoria in which music is performed were subject to the electronic constraints that Mijostyn prefers to apply in his home system.  Would it add to enjoyment of the music?  Would we even like it?  I am not posing this question to criticize Mijo; it's just food for thought.

Almost makes me miss MC.

Soundstage optimization is not easy.  It can be hard identifying the weakest link in the chain that is compromising spacial clues.  For most situations, the most likely culprit is the speaker/room interaction.  You want to really kill early reflections, that is, those with a delay of less than about 12 millisectonds.  If you don't set up and treat your room/speaker/listening position in such a way as to reduce those early reflections to about -20 dB relative to the direct from speaker sound, no other change in equipment is going to fix the problem.  Download Room Equalization Wizard (free software) and learn how to use it.  The impulse graph will be your main tool on optimizing spacial clues.  

REW is a powerful tool that no audiophile should be without.  I once was blind, but now I see.  REW is that good.