I think you have your terms mixed up.  9 o'clock is not very loud on the DT, it's about 2.5 out of 10.  O'clock refers to the knob position relative to the hour hand on an actual clock.  If his amp is on 2 or 3 o'clock that is relative to 7 out of 10.

There are a couple of things I can think of here.  First, the DT has 2 different volumes.  There is a volume for each channel.  If you are using one of the higher gain voicings (2 or 4) then you should have that volume at noon (that means pointing straight up).  The cleaner voicings will probably need to be higher to keep an equal volume level when switching from clean to dirty.

I have never had to put the volume of my DT higher than about 11 or 12 o'clock (4-5 out of 10) and I play with a very loud drummer. 

If this does not fix the problem are you using the FX loop?  If you do not have "equity gain" through the fx loop then you could be losing volume there. Check the volume with the FX in place and without it and adjust the volume on the effect to bring it to the same level.

Otherwise, it sounds like you have a problem with the amp.  Even though the DT-50 is only 50 watts, it gets very loud.  I used to have a 5150 (120 watts) and although there is a difference you should never have problems if you are using a 4x12 cab.

You should not need another 4x12 cab.  You can also try changing the core of your sound a little.  If your other guitar player is using a strat, you use a guitar with humbuckers.  If he's using a heavy bottom end sound, you use more mids and/or highs.  Also, try lowering the gain.  Too much gain will make your tone muddy and not cut through.

Forgive me if this has already been posted by those ahead of my posting as I've not had the time to read all replies to your situation.

The only thing I can subit to you based on your post is that there is no way if your amp and your friends Oranage amp is working properly that you are not blowing him off the stage with the output volume capacity of a DT50.  These amps are monsters when it come to output volume in every possible configuration.

If possible, it would be best if you are anyone else posting used the hand position on a clock to describe where any of your dials are set to.  This would make 9 o'clock position of the line on the knob pointing 90 degrees LEFT and 12' o'clockl is straight up.  I've never ever been able to use my amp on stage louder than the 10 o'clock position in FULL POWER mode as said before - its so incredibly loud.

I use the following on the DT50 channels A & B as settings if I choose to bypass the amps on my HD500...

• Both channels are set identical for my purposes.
• All knobs set to 12 o'clock (Gain, Bass, Mid, Treble, Presence, & Channel Volume) except the Reverb and the Master Volume set to typically 9 o'clock.

Since you are not using anything in front of the amp, choosing on of the four topologies (I've found that Mode III -perhaps because it is the usual Bogner configuration?  It's also used for Mesa as well as Vox amps models) is the loudest on it's own, with modes I (Fender style), then II (Marshall Style), and last mode IV (Engl Fireball style - some have written that this is a Mesa style too, but not according to the specs from Mesa or even Line 6 documentation of what modes emulate which brands of amps) in order of loudest to softest in my testing) will change the output a great deal, setting the Preamp section to Pentode (typically louder) or Triode will affect the tone more than your output volume, then setting the output configuration to Class A/B is typically louder than setting to Class A.

If you try all these different settings, plus make certain your Master Volume knob is NOT pulled out (which is low power mode), you should be blowing the Orange amp away in sheer volume.  If you've done all this or yet to try all these options, write back to anyone here can what what you've done and what you results are. And as written immediately above my post - make 100% certain you have your cabinet plugged into the correct output section.  Make you know what ohm rating your cabinet is if it's not written on the input jack plate of the cabinet.  Not only could your volume be affected, but you can do serious harm to your amp and your cabinet if this is not set correctly.