Do you have anything in front of the amp models in your signal chain?

Can you post a sample?

Don't overlook the fact that it is possible that the HD500 can cause clipping in the headphones. Line 6 recommends headphones that have an impedance of 150 - 600 ohms. Those Sennheisers are rated at 64 ohms. It's not outside of the realm of possibility that's the reason you're not hearing things as clean as you think you should.

I only use a JTV-69, HD preamp models and a DT25 as amp, i.e., I am not sure how much of that experience applies to full amp models and a wide range of real guitar input signal levels. Hope it will help understand what might be going on anyway.

The following describes what I do to get the 'purest' effect and preamp model tones over any range of possible preamp parameter settings. I say pure to differentiate HD digital signal headroom compression/distortion induced tone effects from the selected preamp parameter settings to get preamp specific overdriven tone that you will get from high channel Volume, Drive and other tone stack settings. As far as I can tell (and interpret my measurements) there is an HD or possibly effect/preamp specific soft limit on the modelled output signal level that, when exceeded, will induce additional and increasing compression or soft clipping. At present I do not think that this is a characteristic of the modelled preamp but simply to provide a 'natural' sounding analog-ish mechanism to gradually reduce the output and prevent or at least delay digital clipping within the HD. From everything I checked so far this limit apears to be -12dB, i.e., if your output signal (after any modelled effect or amplifier component with the possible exception of the Mixer) exceeds -12dB you probably are experiencing some amount of compression or soft clipping in your tone that is not due to the quipment model. In addition (I am guessing because there are many more effects than the few I checked), if an HD internal signal level higher than -12dB is fed into an effect or amplifier the respective model will similarly compress or soft clip the output (reducing gain or volume may elliminate that effect, but I have not checked of that). Note, I am not saying this is generally bad if you like or utilize this compression effect.

On the other hand if you just want the modelled clean or overdriven effect of the selected effects or amplifiers I think you would want to choose settings that will keep the models in an operating range that does not induce this additional compression. At present I think you need to keep all inputs below -12dB and choose no effect gain or amplifier drive/volume/tone setting that results in an output signal above -12dB within the HD to acieve this. You may be able to put the Mixer at the very end and push the final output higher to a signal level closer to 0dBFS, the digital clipping point. I have not checked the signal for signs of distortion doing that.

Example:

If I use my JTV-69 on input 1 (input 2 guitar, i.e., None) and select a Strat neck position as guitar model I get -16.4dB as maximum input signal to my preamp with no effects. I can measure this by disabling the amp and setting the Mixer to 0dB gain and channel A/B hard left and right or channel A center and channle B muted via USB and a DAW. If I select the BF Double Vibrato preamp with Drive @ 100%, Volume @ 100%,all other tone controls @50% with Cab simulation off I get a max output signal above the -12dB limit I described above. If you play around with volume and drive settings you will find the full volume range will change the output signal level by about 60dB between a 0% and a 100% setting as long as you stay below the -12dB output. If you go above by dialing in a higher guitar input level or by setting a higher drive you will find the effect of the volume adjustment will get lowered as you go more and more abobve the -12dB output level. With my guitar example I have to place a Studio EQ in front of the preamp and dial in almost -18dB gain to make the BF Vib preamp not artificially compress at maximum volume and drive settings. You can check this by verifying that volume changes will get the full 60dB change on any preamp drive setting.

Obviously the behavior of extreme use modes will only matter if you want to operate in extreme settings. As long as you stay below the -12dB output rule you should not run into this extra compression or soft clipping effect, but unless you actually measure signal levels it is hard to tell whether you get your compression (or even some distortion) from a selected effect, your amp setting, or just this HD built-in headroom compression or soft clipping effect.

Because I just discovered these effects a few weeks ago and I am still making more measurements as well as think about the results the above is really just my current (but not necessarily correct) explanation for what's going on. Your milage may vary ;-) ... and it may not apply to the problems you have, but it kind of sounds like you may be running into what I described.

Good luck,

Martin

I noticed that the OP mentioned that the 2nd input was set to same....... I have found this to cause undesired crossover like distortion is certain amps. Try setting the 2nd input to mic, or variax and see if that helps.