There really isn't anything else quite like the 2150 in terms of features (variable x-over for preamp sub out, USB, etc...). What you might look into is probably an integrated amp from someone else - Marantz, Rotel, NAD, etc... With an integrated, you could use either the preamp outs to the sub and drive the speakers off the speaker level outs. If you connect it via speaker level to the sub and then to the C3s, you will be stuck with the fixed high pass (it is not variable, the variable only applies to the low pass for the sub itself) of the STF-1.
Cambridge Audio might be something to look into also at
www.audioadvisor.com The 540A and 640A are pretty nice little amps.
I'm very happy with my Marantz PM7200.