A quick perusal of the block diagram (loacated at the back of the online owners' manual) indicates that the 1/4" Line input feeds the mic pre just like the XLR input does. The difference is that there are some load resistors to "pad down" the line-level signal. This is par for the course with most "compact" mixers these days, including earlier Mackies and A&H's, etc. So, you ARE feeding the mic preamp from the line input.

Use the insert connection to bypass the mic pre.

Be careful on that...
The recording outs on the Onyx are tapped BEFORE the insert point on the channel strips. Plugging the external mic preamp into the channel insert to bypass the internal mic pre will not feed the multipin Recording outs, nor the Firewire board...

BTW, Dave:
There are companies out there that will do an internal mod to correct that problem if you want to feed the record outs/firewire option with a signal from the insert point (I think). Try contacting Front End Audio (in SC, they're on the Internet) regarding this...

so i'd essentially need a different interface in order to use a different preamp?

any suggestions? obviously a good mic pre doesnt matter. would i be able to adapt down to 1/8" ?

Sorry, Dave, I don't follow the " 1/8" " line of thought here. What's up?
And, as a side note, just curious as to what external mic pre you were considering to buy. The reason that I ask is because the Onyx pre's sound pretty good, IMHO. Not perfect, and the gain pots on mine have to get cranked up pretty good at times, but they're certainly as good (or better than ) a lot of stand-alone pre's out there (think: Presonus, ART, etc.)...
Even as good as my Drawmer MX60. Something to consider...

rockstardave wrote: so i'd essentially need a different interface in order to use a different preamp?

Take Moonbaby's advice. You have better pre-amps in the Onyx mixer than in all but the top echelon of interfaces. Maybe the headroom in the Onyx is not quite in the big league, but you would have to pay a lot of money to get better-sounding pre-amps.

rockstardave wrote: any suggestions? obviously a good mic pre doesnt matter. would i be able to adapt down to 1/8" ?

From your mention of 1/8" I guess that you have a domestic-level stereo mic that you want to hook up to your mixer. If this is one that does not have an internal battery and needs "plug-in power", you would have to use a small battery pack to supply that, and then split the 1/8" stereo jack into 2x 1/4" TS jacks to go into your mixer. I would counsel against going into the XLR inputs unless you really cannot get enough gain from the jack inputs, because of the danger of inadvertently applying 48V phantom power. This would couple through the battery pack and destroy the mic.

rockstardave, if you already have a ONYX why would you want some other so & so's microphone preamp? Those are already considered "boutique quality" preamps.

Now if you said "I want the sound of a API or Neve preamp?" I'd go out and get a couple! It's all right to plug those into the line input on your Mackie. Since the Mackie input is already a good input, you won't find anything better to plug into somebody else's line input, anyhow. But you will get all the flavor, character & timbreal qualities of those API or Neve preamps. If you purchase somebody else's moronic transformer less superduper preamp? Well there is a sucker born every minute I think the saying goes? The only better line input you might find would be on something like a API analog to digital converter. But you're not going to spend that kind of money now are you? Certainly most people with Mackie mixers don't. So go ahead and get yourself an old-fashioned, old school, microphone preamp. Something with transformer inputs & outputs. You could even get 16 of those and have the world's best sounding Mackie mixer. After all, the rest of an API board, excluding the equalizer's, are much different than a Mackie except without the output transformers.

Old-fashioned, old schoolgirl
Ms. Remy Ann David

i was thinking about LA610 or Focusrite Isa One.

is it worth it? or should i stick with my onyx pres and upgrade cabling and mics?

i have average-good mic selection... from Audix OM2s up to MAudio Sputnik.

obviously the OM2s are the weakest link in the chain when i use those, but what about the tube condenser Sputnik? are my Onyx pres keeping me down?