Most any amp you pick in the $5K range will be a sideways move at best. However there are many things you can do to improve sound stage.
For the amp- go to Brent Jessee audio tubes and contact him for some NOS Amperex 12AX7 and 12AT7 tubes and replace the stock garbage that came with the amp. Amperex tubes are not cheap but the sound stage improvement is impressive. Brent should be able to give you some decently priced deals.
Sound stage is mostly defined by speaker and listener position withing the room.
Make sure your stands for the LS50 are no higher than 24 inches. KEF designed your speakers to have a rising response directly on axis and if your tweeters are ear level you are receiving too much treble. Lower them so the tweeters are below ear level.
Experiment with distance from walls - give them room to breathe by moving them into the room 3 feet or so if possible which will improve sound stage depth.
Also if your listening spot is up against the wall you will be hearing too much reflected sound of all frequencies. Try moving your spot at least 2 feet from the wall behind you.
Lastly how many hours on your amp and speakers? Give each at least 200 hours to sound their best.