From the DT25 Pilot's Guide:
Serial Effect Loop – Use the effect loop to send your audio signal to external effects processors and stomp boxes such as modulations, delays & reverbs. Note: Signal path is in series. When using the SEND, audio will not pass through to the poweramp without a connection back to the RETURN.
Cabinet Simulated Direct Out – Provides a transformer-tapped direct output with cabinet simulation for studio recording or live performance without mic’ing your amplifier. The ground lift switch can be used to eliminate hum or buzz.
From the DT Series L6 LINK™ Connectivity Guide:
The DT amplifier’s DIRECT OUT continues to function as an analog output with L6 LINK in use. Use of this XLR output is recommended to feed your “fully baked” POD HD + DT amp tone to an external recorder, mixing console, etc. Since the audio signal fed to the DT amp via L6 LINK is a mono signal, this Direct Out is a mono signal as well. Note that the Direct Out functionality differs based on the DT amp’s LOW VOLUME MODE (see page 4•3) and STANDBY switch settings:
• Low Volume Mode Off, Amp in Standby = The DT amp’s Direct Out is disabled.
• Low Volume Mode On, Amp in Standby = Direct Out signal consists of the “Full” (preamp + power amp) Amp Model with no analog amp.
• Low Volume Mode Off, Amp On (not in Standby) = Direct Out signal consists of the “Preamp” Amp Model with the analog amp.
• Low Volume Mode On, Amp On (not in Standby)= Direct Out signal consists of the “Full” Amp Model and the analog amp.
Based on all the above:
- serial FX loop is between preamp and analog power section. XLR out is (power section output) transformer tapped to give you the "fully baked" signal for the PA or recording.
- seems safe to say that the only time it "may" bypass the FX loop is in LVM Standby.
Can you confirm all your settings when this discrepancy occurs?
Have you tried bypassing the preamp completely and just going straight in to the FX return to see if you get a signal?