ArtPro Split-Mix 4.....looks like a very solid well-built unit. Bought one of these to use to mix bass signal and stereo amp signal for use through headphones: however...... Bought 2 6ft stereo TRS cables. Plugged one into "headphone out" on bass amp. Plugged the other one into "headphone out" on stereo amp. Plugged these two into "mixer inputs" on ArtPro unit. Plugged headphones into "mixer output". I get sound.......but it is very faint even with volume controls on in it turned to max. What do I need now to get lots more volume? Pretty sure I've got everything hooked up as per instructions.... I can hardly hear the stereo amp.