Anyone using Luxman m900u with Audio Research Ref6

     Is anyone using Luxman m900u with Audio Research Ref6? I have it in my system and I feel something is a little off and I'm not sure what it is. My current set up is,

Vimberg Tonda Speakers
Luxman m900u - Amp
Audio Research Ref 6 - Pre Amp
MSB Discrete - DAC
Lumin U1
VPI Prime Signature
Oppo 205

It sounds good really well most of the time and sometimes I feel the synergy is a little off. I'm not sure whether it is my high sensitive speakers or the DAC or the Audio Research. It drives me crazy sometimes. Considering Luxman cl-1000 now. I'm not even sure whether it is Audio Research Ref6. Most of my gears are new though. Just wondering what could cause this. Any suggestion is appreciated.
How much warm up time to do you give your gear? 

PS - Given a choice I'd go all Luxman. :)
Ten minutes warm up time. Although I have some routines like getting up going downstairs, turning on my equipment, brushing teeth, get coffee, sit down and listen, so warm up time happens without a hassle... but the gear sounds great immediately, but does improve over ten or fifteen minutes.

I would say if given the choice of either Luxman or ARC, which ever you choose, it would be highly advantageous to go all that brand. I am and have been completely in synch with where Audio Research is going over the last 40 years, it has just taken me a long time to realize it, afford it, and for them to introduce a really reliable, auto biassing tube amp. I don’t want to fiddle.
“Sometimes synergy a little off”—as in shouty and confused imaging at times/when pushed?

If so, and if you currently have everything plugged into P10 (including amp), you might want to try separating amp elsewhere first. Ime, P10 is insufficient to anchor fully a system like yours. Try P20 or something like Torus AVR16.
I had a system with an ARC Ref6SE preamplifier and a Luxman M900u amplifier, and now I have a Luxman CL1000 preamplifier and ARC 250SE monoblocks and it sounds much better. Driven by an Esoteric 01-XD SACD player, streamer (and a clock). Wilson Sasha 2 speakers.