Spend more money on the amp or the pre-amp?

Yeah. But let's extend it to $8000, what then?

I'm on the integrated boat too. I spent more than $800 building my Pass F5 and that thing is crazy simple. No soft start. No speaker protection. No current limiting. Class AB amps can be built cheaper because you don't need the hulking power supply or heat solutions. That said, you don't need any heat solutions for a pre-amp, and the power supply need only be very consistent and very quiet. 
If all I had to spend was $800 I'd buy a pair of Pass ACA amps and blow the rest on a pre-amp. 

@inna 

$8000 just means I'm building a pair of Pass F5T's and a real nice volume buffer and source selector. 

I think there are some super value low cost amps, like Odyssey and Job 225, not sure about preamps

Outlaw Audio RR2160 Stereo Receiver $799