I'm using the Elrogs as I type this. They've been my default go-to rectifiers and power tubes ever since I got them back in June.
Elrogs makes both 274 and 5U4G variants. I had originally bought the 5U4Gs, but they were backordered. Parts Connexion recommended I try the 274. TECHNICALLY compatible, but one of them immediately popped when I powered on the amp. They went back and I was sent replacement 5U4Gs, which have been flawless ever since *except* that one of the 5U4G will make a physical "ringing" sound every once in a while. It goes away if I tap the tube.
Thomas Meyer (Elrog's current owner and chief engineer) inspected the defective 274 and thinks the filament may have come loose during shipping. He also says that he does not recommend using the 274 in the Woo because of plate voltage requirements (pulling that from memory, will look up his exact explanation later).
Performance-wise they out-perform everything else I've tried (see my signature). By far. YMMV, of course.
As for the 300B, they make 3 variants - ER300B, ER300B-Mo, and TM300B. The TM variant is limited production and reserved for use only in Thomas Meyer's hand-built amlifiers. I have not heard the ER300B, but the -Mo's are outstanding in every regard. If you put them up against the Western Electrics, you hear how slow, bloated, and rolled-off the WE's are. Fine if you want that romantic tube sound, but the sound flabby and lazy. If you put them up against the Takatsuki 300B, they're even more clean and precise, but with far more weight and impact. Compared to the EML 300B, 320B, 520B, etc....they really show off how modern engineering is moving the 300B out of the 20th century. Yes, they're pricy, but worth it (IMO).
Feel free too ask any questions - I'm here to help as best I can.