Okay, so I now know you are using the line inputs of the mixer.
I’ve had a look at this mixer, it’s pretty basic.
The are 4 stereo line inputs, and 2 mic/Line inputs. the mic/line inout only has Eq controls.
Need to clarify which inputs you are using:
If it’s 1 and/or 2, you have a variable gain control, 3 band eq, and a low cut button, which for your use needs to be off.
If it’s any of the stereo inputs, you only have a line level switch. I would use it on the +4 setting, but it would work on the -10 setting, but you need to lower the volume control on the SPD-30.
Also need to clarify, what cables you are using:
(I know you are using jack-to-jack)
‘Normal’ screened 1/4” jack leads?
Leads using stereo 1/4” plus both ends (balanced leads)?
Finally need to clarify what you define as lack of clarity:
Is it distorted?
Does it sound dull or mushy?
You aren’t using any of the effects on the mixer are you?