The Pure Reference Extreme (PRE) speaker is better off with more than an 8 watt 300b. Not every Coincident speaker mates well with the Frankenstein and this point should be made clear on their website. It is a superb SET amplifier but only with an appropriate speaker load match.
IMO the PRE would be better matched with a good quality 845 or 805 output tube it one prefers a SET topology. High quality moderate power push pull tube amplifiers would also be a good choice. The Frankenstein is an exceptionally natural sounding amplifier with excellent purity, it deserves a proper speaker mate. I suspect that the Canadian Coherent Audio speaker would mate very well with the Frankenstein.
Charles