Rhljazz,
I use my sp16 mainly with Bel Canto ref1000 monoblock amps and my larger OHM Walsh speakers.
I would say the bass with combo is lean, but mean as well. :-)
Everything seems there and controlled and articulate and detailed. The absolute lowest octave might have a slight rolloff, but nothing of real consequence I would say.
Overall, by far, it is the best combo of quality and quantity of bass I have ever had, and no other reference systems I have heard in recent years make me feel lacking at all in regards to bass.
I have had some other amp/pre-amp combos in there that shook the rafters more easily, but that was mostly a quantity versus quality thing.
I'm sure there are other better pre-amps than the sp16 specifically, but it is hard to fault on its own terms I would say and at its price point.
I noticed a huge total change in the bass towards what I have today when the Class D amps came in. Much more open, dimensional, articulate yet powerful than anything I used prior.
The ARC/CLass D combo is most likely to come off as "bright" with some speakers that might come off that way overall normally, like the small Triangle Titus speakers I mentioned above. These are by far the most limited speakers I use with teh combo in terms of flat bass extension. Not much below 50 hz there. So things tend to come off as brighter in their case in egenral I would say compared to my larger and more extended OHMs or even my only slightly larger Dynaudio COntour 1.3mkII monitors, which do a good job down to 40 hz or so.
