High gain guitar amps sound very bright without a speaker or impulse response to filter a lot of the high frequencies. If you don’t have the IR enabled properly, or it’s in the wrong place in the signal chain, it’s easy to get a bad sound fast.
Firstly check your convolution plugin is switched on and your IR of choice is selected.
Secondly, make sure 100% of your guitar sound is going through the IR. Sometimes the plugin will have a mix dial, set this to 100%. If you’re using a reverb convolution plugin, set it to 100% wet.
Lastly, the IR must be in the right place in the signal chain; make sure it’s after your amp. If you’ve ever rolled your tone control back with a high gain sound and wondered why it doesn’t get any darker – that’s exactly what you’re doing having the IR before the amp simulation.
After these quick few checks you should be good to go!