Dehavilland Mercury tube swaps

A quick question to all Dehavilland mercury 2 owners.

Its only about tubes basicaly.
Have any one of you experience tube swapping the 6BX7 and the 5U4(5AW4) rectifier in this pre-amp.
I love the unit that i just acquired recently but always looking to experiment.

I noticed quite a few 5U4 or 5AW4 variants for sale on the net, does this rectifier make any difference in the Pre?
Also some older NOS 5U4 are shapped with a bigger glass above the bottom pin support and was wondering if these fit in the Mercury since half the tube is recessed into the chassis.Diameter maybe too big.

- 6BX7 tubes i think are worth eperimenting with, are these replacable by anything else because it seems very hard to find any info on these on the net, is there a NOS that fits the bill or a newer version with a different code?
Original Mercury tubes where 6BL7 tubes but when it was sent for an upgrade it came back with 6BX7 tubes, is there any difference between both?

Any feedback would be greatly appreciated.

Regards, Nick 72