Thinking of changing amplifier to

That's like comparing apples and oranges.  The BHK 300 mono blocks are in a different and higher league altogether. Not to mention the fact that you'd be stepping up to separates.  Even the BHK Signature 250 s, which are a couple of steps below the H390 would surprise you!  I can't give you a side-by-side comparison, but I do own several PS Audio components and I can tell you that the voicing of their components is dead on!  

@curiousjim Going from the 250 WPC with the H390 to the BHK @ 300 WPC will give you nothing in noticeable power increase,  IMHO.  Not sure that I would see the point? The H390 is a wonderful amp.  Again, IMHO.  In my own experience, If you're not going to an amp with double the power, there is little to justify the move.  I happen to think Hegel makes great equipment.  My 2 cents.  

I have seen that it is not solely the watts that are determining the power from an amp. My CODA #16 is only 150 | 300 | 600 watts and it sounds more powerful than the rather powerful Sanders Magtech at 500 | 900. I think it is the power supply that is making the difference.

@bigtwin 

I know the H390 is a very very capable piece of kit, but…

I have a noticeable lack of midrange in my system.  I’ve been following the prices of the used H590 market and for about the same price as I can get a used H590, I can get the 300’s. My thoughts were because the 300’s have a tube component, I might be able to adjust the midrange with some tube rolling. Oh and seeing as I have almost never used the DAC or streamer sections, it might be another reason to switch.

@ellajeanelle 

The H390 is the first integrated amp that I have ever owned, not counting the receiver that’s attached to my TV.  So I am well aware of the differences. Oh and there are many many other factors besides watts that are considered when choosing any amplifier.