So, I have the following problem:

I have a SB16 CT2230 + new quality studio speakers. When I hook them together, I get BEAUTIFULLY clean and crisp sound.

I have a myriad of audio devices from external USB sound card Xonar U7 to a Fiio X1 MP3 player.

I also bought a mixer - the Behringer 802 which allows me to add 3 different stereo inputs and to mix it into a single output + allows me to set the EQ on the fly.

What I want to do is mix the Sound Blaster 16 with any of the above! The idea is to use the same speakers for both the Sound Blaster 16 & any other input device. My laptop uses the Xonar U7 for MT-32 and SC55 emulation, the Fiio MP3 player is handy for playing certain music over games that lack music to add atmosphere to the experience, etc.

If I hook up the laptop and the MP3 player, it works fine. In fact, if I hook up ANY two devices, it works fine - nice, crisp sound with no noise. The MOMENT I hook up the Sound Blaster 16, I ALWAYS get noise from the Sound Blaster's side. I know this because with the mixer I can mute each channel. The moment I remove the other device, the sound from the Sound Blaster 16 turns clear again.

I've tried pretty much everything - different cables, added an amplifier between the soundcard and the mixer (hoping the noise would not get amplified but it does), switching from speaker-out to line-out, disabling everything in MIXERSET (you can set everything to zero, the noise remains), ... nothing helps. EDIT: also tried swapping to different ISA channels. Oddly enough, even with the PC turned off, I get this noise.

Since I KNOW quite a few people use mixers to combine the MIDI modules with soundcards, I know this is possible without the noise so something I'm doing must be wrong. I know too little of the technical aspect behind sound to figure this out myself though. Does anyone know what is going on? (and please don't say "Sound Blasters are always noisy" because I get perfectly clear sound when I don't combine the output with any other device). Thank you!