I use 4 ohm taps with my 6 ohm speakers. That will get you better bass for sure. Also remember, if you have been using the 8 ohm tap for a while, when you switch to a 4 ohm tap, your amp will go through a small burn in process for a few days. It's not as bad as when the amp is new, or if you replace tubes. But it's real and might account for the slightly more rough sound you're getting from the 4 ohm tap.