I’m not surprised about your experience with your Merlins. Mine were a pair of VSM-MXe’s which were a couple of generations older than your Black Magics. The Merlins are rated at 89db sensitivity and 6-8 ohms impedance. That is not a good match with the 8 watt Frankensteins. That combo sounded exactly like you described - thin, lack of bass, and a restricted soundstage. You want speakers that have a high sensitivity rating and with high impedance to get the best out of your SET amps.
My other speakers at the time were Coincident Victory II’s, 93-97db sensitivity and 14 ohms. They were a good match with the Frankensteins; really, it was a night and day difference when compared to the Merlins.
The Victory II’s are no longer made, but have been replaced by the Super Victory which has an MSRP that is less than the MSRP of the Merlins. The Super Victorys do not go down to 20hz, but come close. I strongly considered them. Have you looked into them?
I ended up buying a used pair of of Von Scheikert speakers that are 99db and 8 ohms. Like you, it was important for me to get full range speakers and the Von Schweikerts go down to 21hz. They are hard to find, though, as only a couple hundred were made.
Most of the speaker brands that you are looking at are not appropriate for your SET amps. As for what would work, there are not many SET friendly speakers that go all the way down to 20hz. A few that caught my interest are made by Tannoy, Classic Audio Loudspeakers, and Avantgarde. They are kind of expensive, though.
Also, here is a good starting point for speakers that might be of interest to you that would work with your amps: http://www.high-endaudio.com/RC-Speakers.html#SET . You will need to do some research and sifting, as many of these do not meet your 20hz criteria.