You should contact ARC and confirm that your ARC preamp can drive the low input impedance (10K ohms) of the 260. Per the specs listed on ARC's website, 20K ohms is the minimum load for the 5SE. My guess is the 5SE will have a difficult time driving the 260.
Good advise. Just gave them a call.
Irrespective of what they tell me, I will execute my plan (I already committed to buying an MIT speaker cable). I like my preamp. If it turns out I like the Spectral poweramp (based on what I hear driving it directly from DAC, which many users have been doing successfully, including Adam (Elberoth), but it does not work well with the preamp, I'll just have to decide what I like most (class A mono's + ARC preamp or Spectral driven by DAC direct) there is always going to be a compromise....