Based upon your criteria I don't think that any speaker will do what you describe. Switching to the 9/1 will give you a slightly bigger sound and will keep all the positive elements of the SP100s. Surprisingly, they can be had used for less than $2,500. The bigger Harbeths are better overall speakers, but even on the used market they go for quite a bit more than your budget. You might also consider Monitor Audio Studio 50s (very hard to find on the used market).
I've owned S100s and now own a pair of 9/1.
I've owned S100s and now own a pair of 9/1.