The upper bass of SP100 can overwhelm certain rooms (not necessarily small rooms) and make the speaker seem a bit muddy. Room treatment, particularly bass traps in the corners of the room can do a lot to tighten up the upper bass and improve the perception of more deep bass impact. The other suggestions about digital room correction and possible use of a subwoofer are also very good.
If you like the basic sound of the SP100, you should look first at such corrective measures rather than looking for new speakers.
However, if you are getting an itch to try something completely new, that can open up a whole new range of possibilities and discovery. It is almost impossible to make specific recommendations, given how even small changes can be significant. For example, the same drivers of the SP100 are used in the discontinued 9/1. The 9/1 is a VERY different sounding speaker -- tighter, leaner and clearer (more "modern" sound)-- which is a plus or a minus depending on one's taste. That is why I cannot begin to guess what would be a suitable recommendation for something "similar" but perhaps an improvement.
At best, I recommend that you look at speakers from certain manufacturers whose speakers I generally like that I am crudely guessing will be compatible with the taste of someone favoring Spendors: ProAc, Harbeth, Classic Audio, and Sonus Faber.
