P.S. It’s amusing to read strong reactions to ’classification’. I believe it helps, but should be combined with subjectivity (preference), context (system matching) and nuance (myriad of characteristics beyond the primary sound signature) while making decisions.
Absolutely this should be highlighted, just by the fact that 2 amps are grouped in the same category doesn't mean they sound similar. There are lots of variables as summarized by you, and they'll still sound different even if all the parameters remain the same. Also, each Category in Classification is basically a Spectrum of Signature rather than a single point. McIntosh may sound warmer than Luxman, Naim may sound more livelier than Audiolab in spite of being in the same group ( in some setup McIntosh may trump Luxman and Vice versa). So, With Variability in amp signature added with the complexity of pairing electronics and the variable levels of last 3 aspects like Richness, Control and Musicality ; you have a plethora of options to choose from and a never ending confusion, or compromises in one or the other parameters.
One quality which partly reduces the confusion is to have a sufficient technical knowledge (Rather Acumen) to read between the lines in Amp Specs charts or measured objective data. Measurements Will get you halfway in the amp selection ( either you should be knowledgeable enough or have a Pal who is), Leaving Signature, Musicality, Control and Richness of presentation for Audition with your Speakers to finally decide. Because, In the end you don't want to end up with Amp with great objective data but uninvolving to you on personal level.
A question: What could be non-premium/less richer amps with warmer/darker signature?
Actually, I Meant the Premium in the sound signature sense not price wise, but I get your question. (From the Reviews) the well made Chinese amps like Audio GD, Xindak and Kinki Studio EX M1 Should fit the bill. There are umpteen boutique Manfacturers from Europe and elsewhere, few lesser known brands (I'm not knowledgeable enough, FMs with experience can chip in with brand names) but reasonably priced so you won't have to break the bank. But being in India, shipping and customs kills the price advantage many times.
Tube Hybrid amplifiers, Acoustic Portrait ones come to mind. But don't expect a good resale value with all these options. Also, Benchmark, Nuprime, newer NAD in Class D depending upon your taste.
Well matched Tube preamp and SS power amp combo punches much above their price point; in fact even a Class D power amplifier sounds better with tube preamp.
Another option is using DRC like Dirac ( not everyone's cup of tea).
Yet another option is going DIY with Premium quality components, again knowledgeable DIY guys can work wonders with their skill set and aptitude. Definitely, There's a downside risk but worth considering. Class A Amplifiers rarely sound bad (in fact sound very Rich and premium), with lots of kit options being available ; DIY class A has quite a good favourable risk /reward ratio.
As the Class A/AB amp technology hasn't changed much in Last 2 decades or so, you're better off with well maintained Flagship (Not Very Old) Vintage Amplifiers. Single owner amplifiers with no service history are pretty easy to recommend. Don't buy the vintage duds, they're no better than current duds.
Hopefully I've answered your question.