Welcome to Beoworld ! 
And thanks for joining as a silver member, your support is greatly appreciated.
The basic idea and initial goal of the Beomaster 1900/2400 was that of having a good
quality radio in a wonderful design that was never seen before and would fit in anywhere.
It was never designed to be an audiophile machine - and it never will be unless
completely rebuilt inside.
A chain is no stronger than the weakest link and changing some components to audiophile
grade won't do any good unless the rest follows the same path.
The output stage darlingtons are quite good, even by todays standards, not much to
gain there in my opinion. The same type of darlingtons are used in more powerful
amplifiers and in amplifiers with far better specs than the BM1900/2400 (read: BM4400/8000)
so are definitely well proportioned for the BM1900/2400.
But if you have another pair, feel free to try it. Remember to set the idle
current according to the HFE of the new pair.
The low voltage power supply could be improved, especially the rectifier
(if still original) is below par, quite under-dimensioned.
The rest of the Beomaster is pretty far from being audiophile. One brief look
at the signal path around the source switching circuit would make any HiFi
enthusiast fall off the chair.
But the initial goal was achieved and it's still a wonderful radio today.
One of very few B&O designs, that were never copied by other brands.
The tuner crystals ? Sorry, you will have to elaborate a bit here.
Martin