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
Hi Nick72. I swappped out the 5AW4 for a 5U4GB in my Mercury, and there was a great improvement. The sound tightened up very noticeably. I use all Sylvania tubes right now, but would like to try some other brands. I buy my tubes from Andy at Vintage Tube Services (616-454-3467) whenever I can. He can answer ANY questions you have regarding tubes. I bought the 5U4GB on Ebay for $15 total.
Thanks for the info Yoby, I will look into it.
Regards, Nick
You can also try rolling the two 85 triode tube with some vintage RCA's, which are very affordable if you can locate a matched pair? There's a single tube on EBay for $7.65 cheap fun!
I have a pair of(NOS) Tung-sol's black plate installed in the unit curently.
I think it would be hard to get better on that side.
The output tubes are 6BX7GT and i would love to roll these but i have a hard time locating some.
I have a few rectifiers( 5U4 type) on order for rolling.
a pair of Tung-sol 5AW4GT, a pair of raytheon 5AW4GT, a sylvania 5AW4GT, a vintage Hytron 5AW4GT, also a GE 5AW4GT.
so i think i will be covered in that field.
If anyone Knows where i can get some vintage sylvania or tung sol 6BX7 types please let me know.

Regards, Nick
Hi Audiobugged. You said it. That's the real trick isn't it, trying to find a matched pair (of any brand) type 85 tubes.