Anyone tried a different pre with an NADc370?

I've been pleased with the c370 overall (esp. at the price), but I have been considering using its preouts to connect another pre. My thought here is that NAD's stength is in its amplifiers, and the majority of the 370's limitations lie in its pre section. I'd be interested to hear from any of you who have tried the c370 with other pre's. Or, the c270 which is the same amp.
What have you found that works well with this amplifier? What amount of the sonic signature of the c370 would you say comes from its pre sections v. its amp section? Thanks, Jb3