12ax7 vs 5751

Yes, those are good tubes I'd snap them up if I were you. That being said, you seem to prefer the 12AX7's to the 5751 in your setup (though that might change when your Treasures have completely broken in). If you get them and decide you still prefer the TF's, you'll be able to find someone to take them off your hands (I would).

I have Tung-Sol long plate 5751's, GE triple mica black and grey plates, Sylvania gold 3-mica grey plate, 2 mica RCA black plates, and more, but the only 5751/12AX7 slots I have are in my driver board on my power amp. I find that some of the lesser tubes can sound a little stuffy, but all of these that I listed sound pretty damn good. The Tung-Sols seem to be the cheapest (when you can find them), so that's what I use regularly. My driver board is also designed for 5751's; I've found that 12AX7's just don't sound as good in the circuit. I did enjoy 5751's in place of 12AX7's in the guitar amps I used in the past.

Grayf - I got a quad of the treasures in the mail two days ago. I think they sound as good as my NOS Siemens, Svetlanas, and RCA branded mullards fresh out of the box (and apparently there's about a 400 hour break-in window). I think you'll enjoy them.

Mcpherson -

If you're only considering gain factor, the 12AX7 family falls like this:
12AX7 - 100
5751 - 70
12AT7 - 60
12AY7 - 45
12AV7 - 41
12AU7 - 19

Many circuits can function with any of these, but straying too far from the original gain isn't usually good for the sound. A general rule of thumb would be to substitute as follows:

12AX7 - 5751 / AT7 / AY7
5751 - AX7 / AT7 / AY7
12AT7 - 5751 / AY7
12AY7 - 5751 / AY7 / AV7
12AV7 - AY7 / AU7
12AU7 - AV7

You should always check with your manufacturer. The 5751 and 12AX7 are identical in their limits within a circuit (voltage/amperage) but that's not the case with the others. In most applications you're safe, but you may find it easiest to stick to the 5751 and 12AY7 if you want to try out some different tubes within the family.

Xti - The input making a bigger difference than the drivers is a common conclusion. It definitely works that way in my headphone amp.

From what I know, different models are best from different sources. For instance the best "Amperex" 5AR4's were from GB (they're actually rebadged Mullards). Their best 6DJ8 was from Holland. Also keep in mind that Amperex never actually made a tube called "Bugle Boy". If those words actually appear anywhere on the box or the tube itself, it's probably an eastern knock-off. Those might be fine tubes, but they're not genuine. Anyway, I don't know what source is the best for 12AU7's or 12AT7's. I can say that I've never heard anything bad about an Amperex from Britain, France, or Holland.

Miketuason - If you don't feel like replacing all the tubes, the most important place for the 5751's would be to replace the 2 12AX7's in the phono stage. Theoretically, they'd make the biggest difference there as the decrease in noise would be most helpful there.

If you've got the amp still under warranty, it might be a good idea to just watch out for good buys on some of the 5751 or 12AX7's that have been discussed here. If you go out hunting for specific tubes (and want them now), you'll pay a lot. But just keep an eye out for all of the good ones and try to snipe them at great bargains.