As promised here are my conclusions after about 12 h of burning up my RK 300b
1/day 1 - very technical sound with impressive detail retrieval but missing saturation and warmness expected from 300 B tube
2/ day 2- sound over saturated, where is transparency??
3/ start to sing, big improvement in soundstage and separation, outstanding impression of live like presentation, better timbral accuracy
The triplle thikness glass makes a job, much less ringing frequencies compared to PX4 and consequently - less distorted sound.
Those 300 B are much heavier than PX4 because the glass of the valve is much thicker.
Yesterday among other recordings I listened to Sheherazade - a native DSD recording by Fonè which does not bring musicians to your home but transfer you to Torino Phillharmonic House where the music was performed which is even more difficult.
All spatial cues of the concert are reproduced with uncanny precision, only the smell of the Chanel no 5 parfum of your neighbour lady is missing
Today when I installed again the PX 4 tubes for comparison the sound became more blurred and less precise but a bit smoother than with 300b, I am sure they need more hours.
And the soundstage lost a part of 3 dimensionality and holography so impressive with 300B RK.
Those 300 b RK edition are not cheap , with 23 % VAT a pair cost nearly 1250 Euro here in Poland but if one want the tubes which are stock tubes in Pacific DAC , you have to pay.
I am happy with this upgrade of power tubes in my B7.
Probably will swap to KR 5u4g RK edition as well.