Selecting "No Cab" in POD HD500/X completely bypasses it. Make sure the output mode is set correctly, however. For FRFR use, Studio/Direct will likely provide the most satisfying results, but as always, trust your ears.
The HD's cabs are indeed based on convolution, but we didn't just slap an XXX-point IR in there. There's a lot of clever tweaking, filtering, and optimization to get the most out of our cabs (perhaps akin to what Cliff does with AxeFX II's Ultra Res IRs—within the confines of HD—but I'm not sure), and unfortunately, there's no easy way to apply the same process to third-party or user IRs. Hence, there's no way for third-party IRs to take up the same DSP resources as the stock cabs.
That is very interesting. Thanks for clarifying. The other day, I decided to run the HD in the Loop of Eleven Rack to use Eleven Rack Cabinets "Speaker breakup". The line 6 cabinets always sounded louder with boosted Lows and Highs as if someone applied a BBE sonic Maximizer effect to them which has a dulling effect on mids.
I've always got really good results with my HD by itself, but on almost all of my patches , I always have the BASS , treble and presence well below noon. Line 6 had to add a High Pass filter to the Cabinet to cut the annoying BASS as many complained about the extra bass and highs. Obviously not everyone appreciates the "sonic enhancer" sounds because they tend to make the sound harsh and boomy in general.
But since louder equals better, I think I've been tricked out of laziness as I automatically presumed that line 6 cabs were better because they were always louder and I never adjusted the volume equally!
Conclusion: IMHO and based on what I hear, line 6 enhancement to the Canbinets include: Sonic Maximizer type enhancements to the lows and highs (bad for mids), and some compression similar to what you find in loudness maximezers (limiting); I tested with a peak meter, Line 6 cab was always louder than external IR and it peaked much lower!!
Solution: Not to be mean but to be direct: Line 6 should remove all the enhancements from the cabs and leave that up to the user . The main advantage of convolution is faithful recreations of the Cabinet or the room (for Reverb), why enhance a faithful recreation by "sonic enhancers" or artificial sweeteners. Can we have the enhancers/Sweeteners on the side and instead be able to load our own natural faithful IRs. With the DSP saved from removing these enhancements from the Cabinets, one would think that it shouldn't be an issue unless the line 6 IR is very low resolution and it really needs the enhancements.
Be that as it may, I still stand by the Idea of adding an IR loader that can load 512-points IR even if it takes half of the DSP. You don't like it, don't use it. If you like the sonicaly maximized/enhanced sound, then you don't have to do a thing if an IR loader is ever added. I can only imagine how good the HD would sound to many who don't care for the enhancers type sound. That explains why line 6 doesn't have a sonic enhancer type effect, while other companies do. The sound is already enhanced and any more small enhancement will completely destroy the sound.