Brian, I'll take a stab at this.  First of all in the solid state world there are a lot of integrated amps, many of which are quite good.  The simplicity of a single unit and the lack of additional interconnects are indeed a benefit.  

There is however a lot going on inside of one of these, crammed with a lot of circuit boards, wires, and connectors.  What often suffers and is compromised is the power supplies, just not enough room sometimes.  Class D has improved the packaging issues a lot and heat is way less of an issue, but class D is still not the audiophile choice for many and you are posting on a tube oriented site.

With tube gear it is a different story.  In my case I have a separate tube preamp (Cary) and power amp (Decware ZMA).  The tubes alone would pose a huge issue.  I have a phono front end on my preamp and that account for 4 additional tubes.  Between the two pieces of gear that is 18 tubes.  Where would you put them?  How big would the chassis have to be?

The power supply approach is also very different between my two pieces.  In tube gear, to generate power for the amp and gain for the preamp section the design is really kind of at odds.  Steve's "integrated" amps allow for input selection but he is not generating separate front end gain.  He has excellent separate choices for preamps.  For the record I'm in complete approval of his design approaches.  Tube gear really needs to be separates.

Scott

To clarify, tube amps are all I was asking about; being not interested in SS.

Perhaps all of the integrated amps of the past were SS. I had not thought of that.  

Scott seems to have solved the riddle with saying preamps and power amps need different power supplies.  Thank you, Scott.

Brian

There are plenty of examples of integrated amps using tubes.  These range from hybrid designs where 1 or 2 input tubes feed a SS amp up to all tube designs.  I didn't say it couldn't be done, but often design compromises come into play.  

The Rogue Magnum II for example uses 9 tubes in a single chassis and boasts a MM phono stage.  I've heard it and IMO it simply doesn't match up to Decware gear.  Convenient, yes.  Primaluna, same deal.  I only pick on these because they play in similar Decware pricing markets.  Once you cross 5 figure pricing I assume quality better be there.

I stand by my opinion that separates are better sounding when tube designs are desired.  I also had separate SS gear in the past and I believe the quality difference is less noticeable.  There is always a market for convenience so that is why integrates exist.  In the larger consumer markets separates are in the minority.

http://www.psaudio.com/pauls-posts/the-other-side-of-the-coin-2/

Brian, this is a link a series Paul McGowen of PS Audio wrote on integrated amps.

Have a look!
Zygi