I am not familiar with the sound of Sonic Frontier gear, but, your description of your preference would suggest something other than Audio Research and even BAT. The Audio Research gears tends toward a quite lean sound (much more so than most solid state gear).
I suggest looking at Joule, Conrad Johnson, VAC, Cary, Jadis and Lamm. There are a lot of obscure brands that are terrific, though MUCH harder to find used, such as, Audiospace (a Hong Kong brand that has been putting out great gear for a long time), Fi, Air Tight,and de Havilland.
I suggest looking at Joule, Conrad Johnson, VAC, Cary, Jadis and Lamm. There are a lot of obscure brands that are terrific, though MUCH harder to find used, such as, Audiospace (a Hong Kong brand that has been putting out great gear for a long time), Fi, Air Tight,and de Havilland.