Well I got the RK 300Bs in today and spent a good few hours rolling between those, some Marconi PX4s, and the Sylvania VT-52.
Winner? The RK 300B. The most linear sounding, the least confused, the best mid range, bass and treble. It is very even handed and controlled.
It doesn't have a dreamy, open mid range like say a TJ Full Music 2A3 does, at the expense of decent bass. It has a very refined mid range - not pushed forward or backwards with respect to the rest of the frequency range, but nicely aligned with it.
It's very resolving and detailed and able to present subtle detail subtly, if that makes any sense.
Thankfully, dimensionality is excellent, in the sense that its portrayal of space is isn't overly hyped. It doesn't deliver bass into your lap like, say, an EML 45 solid plate used to do. There's nothing mad going on with it. It just sounds plausible in the manner in which it places images, not silly hyped.
A flip over to my newly acquired Marconi PX4 and we're immediately presented with a rather rougher mid range with more bite, which is fine when you just don't want the refinement of the RK. Good with Nirvana and the like. A bit more of a rocker of a valve, the Marconi.
Bass is tight and well controlled. My bass panels love it due to its linearity and extension. There's a great sense they being really evenly controlled, which is awesome at high volume on bass centric music.
The valve is fast and able to do quick with consummate ease in what appears to be any part of the frequency spectrum. Treble is subtly detailed and sweet when the source is, but sweet when other valves aren't so sweet, if you catch my drift. I don't believe it is coloured, just less splashy and more accurate that other options.
I am not sure about QC, though. There are three bits of mica that should touch the glass at the top of the valve. On one, they aren't touching. I knew it as soon as I picked the valve out of the box and did a flick test. It rings more than the other valve because these bits of mica support the internal structure to prevent structural resonance.
So we have an excellent performance, really, even with one valve slightly structurally compromised. I will inform KR and see what they say.
Until Ked's 242s show up I can't compare, but I will