I am not so sure that RB-1's are under the radar and "PX-10" seems to carry almost as much cache as "paramount" with everyone wanting to have found a PX-10 because they know they are valuable... or by under the radar do you mean they just ride a lot better than people give them credit for? I believe that it is hard to make a really poor riding bike if you are starting with excellent materials, but it is even harder to build a bike that is exceptional and even then it is often exceptional only to the right sized and weighted rider.
The one vintage frame the surprised me the most was my Gazelle Champion Mondial... The ride was exceptional. And while I would consider marinoni to be an "under the radar" brand for the exceptional quality, I have never truly felt they rode exceptionally better than a good quality double butted frame.
__________________
1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear