The main issue that I see with your system is that the Koetsu Black is too good a cartridge for the Rega P5 - it is starting to show the shortcomings in the arm. This is a common mistake made by many folk on this forum. Its like putting NASCAR tires on a tractor and expecting it to go round a racetrack.
I own a Koetsu Black - have had 2 plus Koetsu Red's Onyx's etc - they sound best with tube preamps loaded at 47k exactly as Allen Wright suggested.
The EAR 834 has a natural easy sound and you would not be disappointed. However my suggestion would be to get your Magus serviced and get the coupling capacitors upgraded - this will get a big improvement out of the existing preamp.
The passive linestage in the MFA should not be a problem with dynamics because the valve power amps you are using have a very high input sensitivity.
I own a Koetsu Black - have had 2 plus Koetsu Red's Onyx's etc - they sound best with tube preamps loaded at 47k exactly as Allen Wright suggested.
The EAR 834 has a natural easy sound and you would not be disappointed. However my suggestion would be to get your Magus serviced and get the coupling capacitors upgraded - this will get a big improvement out of the existing preamp.
The passive linestage in the MFA should not be a problem with dynamics because the valve power amps you are using have a very high input sensitivity.