@ Just Startin'
I also have a DT25 which I love. Given all the comments on this forum about replacement tubes I wanted to get a clear and concise response so I opened a support ticket with Line 6 and here is what they said.
1 - For the 12AX7 preamp tube you can buy Electro Harmonix, generic chinese, Ruby Tube, or Sovtek. 2 - For the two EL84 power tubes you must use Electro Harmonix or Sovtek (Electro Harmonix owns Sovtek)
While I have read about other brands that people have tried...my recommendation is that you stick with what Line 6 recommends. Also if you don't have training on amp repair and components installation....stay away from doing it. You can damage the unit or get hurt.
Peace out
JG