I was also not satisfied with the Sound of my POD HD500X connected per L6Link to my DT25 Stack! The sound of the pure DT25 Stack is fantastic, but connected with the POD HD it sounds too muddy and dull with the same models - so I did some experiments to find the reason why! (and sorry for my bad english!)
All Firmwares are actuall!
1) The first thing, i learned: If the cabinets in the POD HD500X AND the DT25 (edited via DT Edit!) are switched off, the Amp with/without HD500X sound identical!
(I switched the Input to "same" and used only the preamp models)
2) Without Cabs, it makes no Sound difference in switching between the different Output modes: (Stack PWR Amp, Stack Front, Studio/Direct...)
3) activating the same Cab simmulation in DT25/HD500X makes the big difference - so either the Speakersimulation or the Output Modes alter the sound differently
- and here is the evil, i think!!!
4) If i use a Signalchain in the POD HD500X with no Amp in it, it switches the DT25 via L6Link to Ch "B", which gives me the pure Amp Sound - identical to connect the Guitar direct into the DT25 - so this can be used for "pure" DT25 sounds or for only pre FX out of the POD HD500X!
For equal Gain, the Master in the POD must be at 2 o'clock, because now it works as a Gain-Stage BEFORE the DT25 Preamp!
5) If you set the L6 Contol (POD HD500X Setuppage 10) from "Amp A(Ch1) to "Ch3", you can program two of the Footswitches with MDI CC19 to change between DT25 CH A/B and use also pre effects (POD Master to 2 o'clock!) Use the Reverb of the DT and not of the POD, otherwise you will distort the reverb via the preamp!
6) Additionaly the "Boost" function in DT Edit has Impact to the DT25's Volume (Midi CC74(Ch A)/117 (Ch - not documented!)
But I want to have Pre and Post Effects and the DT25 orginal Sound, so i did some measurement:
I placed a Neumann KM84 Mic 25cm before the Speaker of my LIne6 1x12 closed Cabinet and feed a pink noise Signal via the Aux Input into the POD HD500X.
Then, I used a FFT-Analyzer to view the Spectrum and switched between single DT25 and DT25/POD HD with the same Models.
Next i tried to correct the frequency response with the parametric EQ switched at the least piece in the PODs Signal Chain. Unfortunately it has not enough capabilities to fix all differences, but i do the most!
I measured the 2x12 Blackface Dbl, the 4x12 Brit T-75, the 2x12 Silver Bell and the 4x12 Tread V-30 Cabs!
Here are the results:
2x12 Blackface Dbl: Low 57%; High 68%, Freq 100%; Q 39%; Gain 72%
4x12 Brit T-75: Low 58%; High 44%; Freq 49%; Q 75%; Gain 19%
2x12 Silver Bell: Low 45%; High 50%; Freq 41%; Q 85%; Gain 13%
4x12 Tread V-30: Low 50%; High 45%; Freq 43%; Q 61%; Gain 39%
The sounds are now much more close to my DT25 Stacks Sound, i would say for me, about 80-90%, dependig on the Cab!
I don't know, if this settings are fitting to the DT25 Combo. Maybe Line6 tries to compensate the open back frequency response in the Combo Version differently!
Maybe, sometimes i try to measer all the other cabs to.
I also hope, Line6 fixes this problem in a new Firmware for the POD HD500(X)!