Silverline used to be tube friendly some years ago, but the current stuff is not. There are two design paradigms used in speakers, and the Silverline stuff now subscribes to the idea that an amplifier must be capable of constant voltage, regardless of load.
There is no tube amp that can do that.
BTW it is not important that the impedance curve be flat. It **is** important that the impedance curve be high enough, once it gets above a certain level, it can vary quite a bit without ill effect. FWIW, even if you have a transistor amp, the speakers with higher impedances are going to sound better because while many transistor amps can drive 4 ohms or less, that is not the same as saying that they are sounding their best. That comes with higher impedances. IOW there is no good argument for 4 ohms in high end audio, although there is for sound reinforcement.
4 of the speakers in Tvad's list seem to have been designed using OTLs- Audiokinesis, Coincident, Merlin and Classic Audio.
To add to the list of tube friendly speakers:
Quad 57 and 63, but not the later models
any 'full range' driver like a Lowther or PHY
all horns, with the exception of the AvantGarde Trio
High Emotion Audio
Vandersteen
Wilson Audio
