diablo 300 vs arc ref 5se + pass labs 350.5 - anyone has opinions to share

That is going to be a tough one to answer, as the sound quality of separates will also depend on the quality of the interconnects and the two power cords

One main point would be having a tube preamplifier which generally produces a wider and deeper soundstage.

The Pass Labs amplifiers are either beloved or not so much for some they are a bit too warm and lacking in the ultimate definition of the class of amplifiers from Gryphon, T+A, Vitus, Solution etc.

You have an excellent set of components that are well matched perhaps improving your digital or analog sources or making other improvements might get you better results

In our opinion the Gryphon Diablo and any other uber intergrated will be different but not necessarily better than what you allready have. 

The only way to know for sure would be to do a side by side demo, and then I wouldn't rule out also listening to any other contenders for uber intergrateds either.

For example adding Critical Mass footers will greatly increase your macro dynamics, lower your noise floor and increase overall resolution.

There are really many avenues you can try to take your system to the next level power cords, power conditioning. etc.

Dave and Troy
Audio Doctor NJ

Gryphon Diablo 300 is a hot selling amp these days. But it is not for everyone and it has passive preamp section, something that I personally do not like much. I strongly prefer active preamp. But it is Gryphon and it is great.
Pass won't have speed, resolution and dynamics of the Diablo 300.
Audiotroy is right - it depends on many things starting with speakers.
My idea - tube active preamp into line-in input of Diablo 300. Should be electrically and musically matched. And separate tube phono stage, of course. 

The latest Gryphon Diablo is the most big power integrated I have ever heard .my brother owns it.i owned a Audiostore until 08 
in the UK and heard the best from both sides of the pond .my brothers unit is the latest model Fantastic integrated done right with no limitation can be better  ,far shorter signal paths internally 
no extra interconnects,as well as power cords. And they even make a exception internal dac which was several grand but truly 
packed with musicality and resolution , the pass 350.5is very good 
with their upper end preamp ,but as as complete or musically as effortless then the Gryphon from my observations, that being said 
cables, and speakers too can make or break a system unless I heard all 3 of them with the same equipment I could never make that call for sure.all the Flagship Danish integrated roughly100wpc 
class , which I heard once but great those 2 best integrated s I have heard. Each one has its own strengths especially vacuum tube vs solid state.