Here's some opinions from headwise
If they were on E-bay, and say PQ on the box flap, they are fakes done in Hong Kong. They are Sylvania 6922, which if they are selected, can be OK. Problem is that a very small percenage will be low enough in microphony for pickier pre-amps.
Depends on the code etched on the glass. I seemed a few Tungsram E88CC labelled as Amperex with PQ sign but without gold pin. If the first character on second row (etched code) started *, then is made in USA, New York; If it started a triangle leaned to the right then it's a Philips Heerlen plant. I hope these help.
I have had the X-cans for some time and hated the Philips from the get-go! I went w/ Amperex a long time ago and have been very satisfied. However, I have tried about a dozen different sets since then and can add this to the mix. I found that the Amperex A-Frames worked best on this unit. Had a deeper bass w/ richer midrange while still being extended but never bright on top.
Tried the good RUSSIAN tubes from Upscale & they really sucked compared to the Amperex or Mullards. Much better than the stock tubes, but at $25 a pop you can do better. The Russian tubes suffered from a lot of top end shrill while being much better w/ bass response.Still too much like the stock on vocals for me.
Have also plugged in 7308 white labels 7308 orange labels,6DJ8 & 7DJ8 Bugle boys, US White labels, 6922 PQ & Mullard 6DJ8 as well as the russian and some older Philips tubes. I still feel the A- Frames were better in that the round getter model were still just a touch too extended on top.
I use theX-PSU power supply & have Grado SR 325 as well as Senn HD 580 Cans.
One more point to ponder. WHAT INTERCONNECTS are you using? Found the X-Cans to be very susceptable to ICs. I tried JPS Superconductors, Cardas Golden Hexlink 5C, Cardas Cross, MIT 330 II, MIT 330 HE, Wireworld Atlantis II, TMC Yellow, TMC white, Kimber KCAG, Harmonic Tech Pro Silway MKII, & Harmonic Tech Truthlink. FAr and away the best sound was with the HT Pro Silway MKII, with the MIT 330 HE and Cardas Cross a real close second. The HT Truthlink is probably the best budget choice though as it is very close to the others just lacking a bit of detail while maybe being a touch stronger on the bass w/ some recordings. Plus they can be had for only about $150 delivered on a good day! :-)