Krell Preamp function?

For Krell Showcase, HTS and S1000 surround processors, they all have Preamp (button) function, what is the difference between these models?
From their manual description, they all bypass digital circuit. But HTS's brochure had emphasized its analog throughput ability. So I am confused here.
If I want to purchase a system recently, which one has the best "audio" C/P right now? Second hand HTS+TAS, or S1000+S1500?
I am using Atlantic Technology System 450 THX + Pioneer AX5-AIG right now.
(I will conside adding a tube pre-amp for my stereo system.)
Thanks for your suggestion.
I have owned both the HTS and now the S1000. The preamp button, as the manual describes bypasses the digital processing circuitry. The idea is that the digital processing somehow degrades an analog signal, such as a phono.

I never could tell much difference with it. What I could tell a difference with is the S1000 versus the HTS. The S1000 has a much better sound. Whether listening to music or movies, the difference is noticeable. I have mine hooked up to various Krell amps, so I can't comment on the S1500.
Since you want the best 2 channel performance possible you should consider the Krell KAV2250 for L & R and then depending on your budget a 3 channel amp for center & surrounds. The S1500 is a great amp for the money but in a 2 channel scenario the extra power from the KAV2250 will be worth every penny. If you were more theater than 2 channel then I would say the S1500 would be more than enough amp.