12ax7 vs 5751

Mcpherson
I own Focal JM Lab 936 non Berylium tweeter speakers and it took a combination of things to get the tone I wanted. You have to ignore all the negative crap you will read about trying to adjust the tone of your system. In my mind it is my system and I want it to sound the way I like, not the way some one else delares is correct.
The tubes you want to warm up that tweeter is most likely going to be a good old stock 12AX7 Mullards from the 50s or 60s that you put into the pre amp. The Blackburn long plate in particular. I think Brent Jesse at Audiotubes.com may have them. These are warm tubes, but be aware that almost any British tube is going to lean towards the warm side. If you can roll input tubes into the HK again look to Mullard for the best results.
The other things you want to do- although this will cost quite a bit of money- is to get Cardas "Golden" whatever interconnects, I have the Golden Refs. These copper interconnects really make the Focals sound great. I also suggest good copper speaker cables. Again ideally Cardas or Jena Lab but both do cost a lot even used . I would never the less look for them to show up. I lucked into some used Jena Labs and the sound is fabulous.
I happen to have an AE-3 Pre Amp by AES a Cary brand which uses 6SN7s which are delightful tubes for smooth and rich tone, it made a world of difference in my system. Good Luck!

Mechans
Thanks for the information. I am assuming you had a similar impression of the Focal sound as you started tinkering around with tubes and cables?

Grayf I am fairly new to tubes also and enjoy tube rolling a lot. Friday night I had a real big problem. One of my Gec CV6091/A2900's went out and took a Ei KT90 with it (but didn't know it right away). So I replaced the Gec with my Brimar yellow T's and the KT90's with my Tung Sol 6550 black plates. Sat morning I decided to verify the KT90's and sure enough 1 was bad. I didn't know it at the time but when I put the 6550's back in and damn the Brimar took out 1 of my 6550's (all in the same positions). I never thought anything like this could happen. But it did. I have all stock tubes back in now except for my Rca 5751 triple mica black plate. So it was a very sad and more important very expensive weekend for me. I'm still confused because I have had output tubes go before and that was it. Anyhow be careful should an input or driver tube go.

Forgot to mention the lesson I learned is in the future if I have any input or driver tubes go bad to have them checked out with any output tubes in the channel.

Yes, as a general observation, British tubes do deliver a warmer sound. There are makes of other nationalities that will also do the same. For a warmer sound, I like Amperex Bugle Boys (Netherlands). These tend to work reasonably well in all applications.