Peachtree audio iDecco - not hearing any difference with the tube on or off.

So I bought a used iDecco and I am using it as a DAC preamp. It is connected to an Onkyo equalizer via its Line Out and the equalizer is connected to my Artcoustic PA300 power amp. I have no complains about its sound as is, I am extremely happy with my current setup, But I noticed that the tube does not really have any impact to the sound? Is it because I am using the iDecco as a DAC and not as an integrated amp? I havent had the chance yet to connect the speakers directly to the idecco and test the tube. I also just recently changed the tube from 6nj1 to Amperex orange label ECC88/ 6DJ8. I just cant hear any difference at all whether the tube is on or off.
Jepoy, my speakers and sub are connected direct to the Peachtree Nova 125se. I run without the tube....higher signal to noise ratio too so that might be the difference I heard. as always, let your ears be the judge
thank you all for your time. Ill try plugging the speakers directly to my iDecco this weekend and see if the tube would make a difference.
I easily hear a difference and it is rolled off highs, less attack and decay of notes. Works well for edgy digital.
I emailed peachtree just shortly after I started this discussion. Some guy emailed me back and told me that the iDecco will bypass the tube if I plug the power amp via the line out, but it will not if I use the pre out. No wonder im not hearing any change. I used the line out because my power amp already has its own left and right channel volume control.
I can finally hear that slight change in the sound. Im not sure though if its made the sound better, maybe because the tube is still new.