Rolled some 12ax7's recently...

Fbi, Nsgarch, I have Svetlana 12AX7's in my modded Jolida JD100 CDP, and am now thinking of trying the 5751's. Nsgarch, you mentioned the 5751'a are spec'd at 70%-80% the gain of the 12AX7's. I'm presently using a set of Rothwell 10db attenuators with the Jolida to help with the gain. Sounds like the 5751's will help a little more in that respect.

Best 12AX7 I have tried is the very rare and expensive CBS/Raytheon D-getter 7729. I also have a pair of O-getters that are close, but avoid the later CBS/Raytheon with the O-getter and 7729 etched in the glass. These last ones were actually made by GE and sound rather ordinary. Once I heard these I sold my Telefunken ECC803S, a nice tube but thin and lean tonally in comparison.

Abucktwoeighty -- I can't say for sure about your CDP application, but let's just say don't count on less output because you're running 5751's ;-)

If you compare performance (tube) curves between a 12AX7 and 5751, you'll note that the curves are essentially the same until the upper limit of operation where the 5751 drops of some. HOWEVER, I don't think any circuit designer would use either tube where it would be driven to the limit -- there would be other tubes to choose from.

Also, keep in mind that the 5751 was originally developed as a 'ruggedized' tube for the military - so it would have to perform the same as any 12AX7 it would replace.

Just finished listening to 10Ms compared to Bugle Boys.

Mmm...

They are both detailed, clean, open, great with male and female vocals, lots of air, great timbre, dynamic, and involving. I would give a slight edge to the 10Ms overall for slightly more dynamics and slightly better presence, perhaps wider and more depth to the soundstage, too.

However, in light of the current price differential (BB $145/matched pair vs. 10M $350/pair), I would go with the Bugle Boys. Since the performance of these tubes is so damned close, I do not consider the 10Ms worth more than twice the price of the Bugle Boys. I wouldn't consider them to be worth 10% more than the Bugle Boys.

It really was close! I went back and forth for 3 hours being enthralled by both tubes! Honestly. I found it hard to decide. I played a lot of music including Stairway to Heaven, Rickie Lee Jones, Paganiniana - Water Lily Acoustics, Madman Across the Water, some from Goodbye Yellow Brick Road, Mozart's Requiem - Harmonia Mundi, some from Timbre and Solera - Opus 3, September 15 (from As Falls Witchita...) and Lonely Woman (with Charlie Hayden and Billy Higgins) -- Pat Metheny, Diamond Dogs and Ziggy Stardust -- David Bowie.

Again, I appreciate any feedback or comments -- it is very interesting to me; especially anyone with experience with both tubes.
Cheers!