Everyone's opinion is going to be different, because everyone's system is different and the sound everyone's chasing after is different.
I personally prefer Amperex tubes over Siemens in most cases. I have over 600 of various 6DJ8/6922/7308 tubes. You name it, I probably have at least a pair or two in there. I've tried different brands, but always come back to Amperex.
I had purchased my ARC LS-25 Mk I with GNSC Reference mod (used) a few years back from a dealer. When I contacted GNSC, they told me the mod was done less than a year before. With the mod, the previous owner had purcahsed a quad of 1960s Siemens E88CC. They sounded nice, but seem to highlight the midrange more and not as extended at top and bottom. I ended up with a quad of Amperex 7308s, then a combo of Amperex ECC88/Telefunken ECC88.
In my PH3SE, I had been using Amperex 7308 USN white label for a long time, recently just switched over to some Amperex white label E88CCs.
I would suggest you try both the Amperex Bugle Boys and the Mullards and see what you like and not like with each. Then go from there.
By the way, I had spoke to ARC regarding tubes for the PH3SE. You don't need matched pair tubes. There are two sections within each tube. Each channel is using one section of each tube. So it is more important that each tube has matched sections. You could actually use three different brands to further "tune" the sound if you want.