Hej, Thanks for the very warm welcome
I have no issues with AKG-k240 on my computer listening anything basically. I also have ATH-M50X, which seems to handle better but I do not like closed headphones really. ... ...
At 38 ohms the ATH-M50X is a closer match to the 32-ohm minimum headphone jack. And there are a few Rumblers here that like them. But, based on those liking the AKG-k240, I wouldn't expect the 55 ohm phones to be so bad.
... ... Sennheiser HD-600 says it is 300 Ohm, would that require a headphone preamp to boost the sound? I don't know much about the Ohms etc to be honest, just curious from research I made since "Internet" says you need to boost anything above 100 Ohm.. ... ...
I'm certainly no expert, but it does appear that they could benefit from a preamp.
... ... I actually had my Rumble 40 for around 4 years now, and it is a very good home practice amp. I am sure I have not brought the best out of the Rumble I have to admit (used to just keep it at a single setting), since it has all the nice options. But recently I started play more and more with settigs etc![]()
You might want to try some the settings we've collected in the Rumble Club TONE POOL section of your Rumble Club Wiki page. Best wishes! 
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