I think it would be hard to specify a brand of under the radar mid power (25 - 60 wpc) vintage receivers today. People have wised up to the quality and value of those models.
Nikko might be one of last under the radar brand of high quality receivers. Their equipment is as good as any of the other brands, but being less popular, some people still don't know what they have and sell it for good prices. I once would have suggested Realistic, but even some of them are over priced. It's best to go with a case by case search. I've seen some Kenwood, Sony, Sansui and Realistic that seem be be reasonably priced. Not so many Pioneer or Marantz though, Two of my favorites. Don't be afraid of off brand models either. Even brands like MCS, and some Sears/LXI models were pretty good, just research the model in question.
I do see plenty of lower powered (12 - 25 wpc) models however from nearly every reputable brand that would do a fine job for many. But IMO, Your best bang for the buck is a reasonably priced, mid power unit. There out there, you just have to be patient.