It's all in the engineering. If the designer has good ears, and good understanding of subtle electronic principles and knows what to do to fix the problems that occur along the design path then they can achieve very good performance whether integrated or separates.
There is a lot of black art in making feedback and current sources and mirrors work well. Removing out of band IM distortion, eliminating crossover distortion. It is all possible and when it's all fixed you have great sound. It doesn't have to cost a lot of money in parts, but the development takes a long time to get the refinement we want.
I think there are some strong benefits to integrateds. Eliminating the wiring and connectors. Eliminating an extra set of input and output caps. Share PS, smaller space. But the power output is usually limited by heat dissipation and power supply size. Above 100-200w I would look to separates. If the class B crossing distortion can be reduced without a ton of class A bias, then you can make more power in a smaller integrated box, but that's not easy.
My little 15W Samet I15 integrated has displaced (stereofool's loaner) AR Ref1 and VT100 that I was using before. I don't miss them too much. Well, maybe a little.
It has been continually refined for 6 years.
The
Sim Audio Moon Evolution 600i integrated amp intrigues me. 125W. They have a 175W also. Sim's not gonna be cheap.
The
ARCAM A19 is getting good reviews, appearing in European shows with the little KEF K50 speakers. Whether it sounds good with anything else I dunno, but Arcam stuff usually pretty alright. The A19 is from their top FMJ series.
AR's tubed
VSi60 is very nice, but limited power. I heard it with bigfish a few years ago playing Wilson Sophia2. It was excellent. Again, not cheap, but good stuff.
There is a lot of crap in the low/mid priced integrated market. It all hides behind the "good for the price" excuse. Exposure is an example. I bought one just to see if it was as good as Samet I60 integrated, because the reviews were so over the top. It was built like junk and sounded significantly less impressive than the I60. It lists for $2000!! To me that's a ripoff, but they have to pay the distribution and insurance and factory and employees and insurance. So that's what it costs. You kinda have to shoot for the best brands to find good value in that price range. But then you're not in that price range anymore! oops! haha
But you get the benefits of an integrated amp that sounds as good or better than the low end separates.
The used market will offer some great integrated options.
http://www.marklevinson.com/Products/Details/33http://www.stereophile.com/integratedamps/997classeetc.
The link to Agon's integrated amp listing confuses the forum software because it has a close bracket in the link, so you'll have to do that one yourself... But there are some pretty pickins there from time to time.