Generally, 2x4s are white pine or doug fir, usually 2x8 and up are SYP from good lumber yards. If your concerned about the brightness of maple, turn the tone down on the guitar or use a different pickup imo. The neck is the last place I'd scarifice structural integrity to gain warmth of tone.
From my experience, walnut is not even close to the brightness of maple, more in the range of mahogany.
I have a few guitars that are solid maple and a combination of maple and northern hard ash, bright as can be, and even with a SD JB in the bridge position they are great for cutting through the distortion and maintaining clarity on the lower tunnings. If I have them tuned to standard E I role the tone knob back a bit and they aren't overly harsh.
Just make sure any SYP you use is dry. Most that I find from lumber yards is construction grade, more in the 15% to 20 % range and not 5% to 8%