Agreed, the P6 is probably as good as it gets for under $1,500, or even $2,000. I think you worry too much about the importance of preamp sq though. I have my LS50 hook up most of my preamp/amps and I have long concluded that even a $599 AVR performs just as good, though Placebo does work differently on different people.
I hate to say this, but you seem to keep searching for integrated amps and now preamps, so presumably power amps too for SQ improvement, when at the price point of what you have been considering, what contribute most to overall sq is the music source and speakers. If I want better sq, I would replace my existing music cds/sacds/dvda/digital files with better (recording quality) ones. I learnt this myself after spending way too much on my separate gear. I may continue to do so (still considering that little Benchmark amp, now that the Bryston is sold) but it won't be for sq improvement.