Going active shouldn't be a big deal. The classic "LM386" op amp from
Cell Phone Shack should do the trick. They used to have a package that had four of them on a 16 pin chip that we used to use for building
distortion pedals.
Impedance matching is the real key here. I've used a terminating
resistor across an input on a 386 based
circuit, but audio quality wasn't a big issue. I'm thinking too though, that your pedal will be looking for guitar-type
impedance on its input, so you'll need to match at the output of your
mixer as well.
If you really want to properly mix things, you should just get on eBay and locate one of those little pocket mixers. I've done quite a
bit of prototyping audio circuits and even going through one of the online suppliers like Mouser, it can get pretty expensive. I once spent over US$50 to
build a headphone amp that had 1/4
watt out and sounded several orders of magnitude worse than a $30
unit I found on eBay and didn't have as many features either.
But, if you really want to solder stuff (which I frequently do) by all means, get yourself an assortment of resistors and
trim pots. Do some calculations so you know pretty much what you want, then when it doesn't work, keep swapping things out till it does. Hopefully you'll find many hours of enjoyment with burnt fingertips and
smoke curling directly up your left nostril while the gear you're building the
widget for sits un-used in the corner. (I'm not mocking here at all, in fact, I think I need to go sort through the junk box and see what I can
build.)
One last thing. There's a really good tutorial about the use of op amps here:
CMoy Pocket Amplifier. Once you get the gist of how they work, you can download the spec sheet for the one you want to use, look up the external component values for the
gain level you want and off you go. I wish I had read this when I was messing around with op amps years ago. You may want to start out by actually building the guy's little
circuit to get used to it, then you'll have a cool little practice headphone amp that fits in a mint tin! Dang!!! Where's my soldering
iron!!!