If you have an active bass, are you maxing out your treble, by any chance? If so, try setting the bass and treble in the middle position- i.e. "flat" (assuming it is a boost/cut circuit). If you have boost-only, set everything back all the way. Now boost the bass 50%, the treble about 20-25%. Try it there, if it is still a brightness issue, cut some treble. If this gets you closer to the sound you're looking for, great. Check your amp settings, too. The extreme "smiley" EQ isn't actually all that great for slap, IMO. Too much of the sound lives in the middle, and by negating it through cutting the mids, you lose perceived volume. Run everything on the amp flat to start with, and try something novel- instead of boosting what you're missing, try cutting what you have too much of. So often, we start the "Boost Wars" on ourselves trying to get our sound. Got the top, but no mids or bass, we add bass and mids. Now it sounds "honky" and "boomy", so we bump the highs. Now it sounds brittle, so we bump the lows. Now it sounds like an elephant fart breaking glass, so we bump the mids...and so on until we run out of EQ. Not so good. Start with equal amounts of all three- highs, mids, lows...if it is too bright, try cutting highs. Too middy, try cutting mids. Too boomy, cut lows and a little mid. See what I'm getting at, here? You'll end up with tons more headroom, to boot!
I used to try to cop Marcus' tone. What I didn't realize for a long time is that he doesn't really have an extreme EQ setting. His highs are also not boosted very much, yet he gets a clear, snappy sound. What that seems to be is the natural zing of the roundwounds. He boosts his lows a little bit, to add girth, and his mids a little bit for definition. To my ears, his sound is one of the great "slap tones" in music. But I was also subconsciously trying to nail Mark King's tone at the same time- his EQ is definitely different, with tons of top boost and bottom boost. I have found through many years of playing that his tone is great for some stuff, notably Level 42 material, but it suffers in a band context- I have no perceived volume due to lack of mids, and my fingerstyle lines get buried, completely blurry and indistinct. So I live in my mids and the lows and highs are adjusted to the room I'm in. That approach works pretty well for me, you may be in a different situation. But it is at least worth a try.
