Hi n brio,
As far as I understand the Amperex actually built the Phillips tubes and are the same and a way to get vintage Bugle Boys at a lower cost. I've heard Telefunken, Mullard and Amperex referred to as the European sisters. While in like the Tele's and Mullards I've simply never cared for the Amperex. I own the touted early 60's Amperex Mediacal Grades, 1959 Halo Getter Bugle Boys and the rarer earlier long plates with square or D getter. In my room in my rig they are highly dynamic and energetic yet have a pinched and confused sound stage with very little spacing and air between the various sonic images. A complete deal killer for me.
If you haven't read this review you should...it's outstanding although I can't agree with the blanket assessment and indictment of all 12AX7's his description of the 5751's is spot on:
https://www.audioasylum.com/scripts/...oes-tubes.html
In the last 2 weeks I purchased a quad of Sylvania 5751WA TM BP's on ebay for approx $300.00 they are very good. These are my go to tubes....very focused, very balanced top to bottom, very musical and in my rig, the most 3D holographic tubes I've run. These cost 30% of my second favorite, the Mullard Master Series 10M's.
Another great source for suggestions and information as well as purchasing tubes is Brent Jesse....I noticed he has Sylvania TM BP military versions available:
http://www.audiotubes.com/ I use his website as a reference on a regular basis.
I live in the Phoenix area and have purchased dozens of tubes from a local dealer named Dave and calls himself Arizona Tube supply....have fun. Your pre amp is extremely tube rolling friendly and will respond well to all the different variations.
Have fun!