You have already identified that you like your set up - the Kef is a great speaker, however you want to expand the soundstage.
In order to maximize your existing set up and achieve a better soundstage with your max budget of $1,500 the best path forward (as others have pointed out) is investing in upgrading what you already have by:
1. Adding another Vandy Sub.- used - Your adding another L&R diver to the kefs, with a dedicated power source. Expands the Mid/ Low range.
2. Investing in Power delivery. - used,
Think of it as another component to your system.
Will add detail,Imaging, Depth Texture - maximize your amp preamp performance.
3. Look into upgrading your crossovers/recap etc. - Cleaner, efficient presentation.
And I don't even think your budget is going to cover that, but its a solid move towards getting to be where you want to be.
Investing in different speakers in your budget is a lateral/backwards move that will not solve your issues. You want to make investments that move you forward towards achieving your goals.
Good Luck