Currentyl I use the Okko Twinsound with the Alchemist. Booster and Overdrive in one box, can deliver dumbelesque smooth overdrive as well as british AC30. The booster is great as well. Can boost the amp or the overdrivesection of the box. Very dynamic, does not cut the bass or any other frequency. Check it out, they are handmade. http://www.stompbox.de/
My fave pedal to use with the clean channel for the heaviest killer distortion is a cheapo EH Nano Metal Muff. It's ungodly kool. With the gain channel, I'll use a Fat Boost and a Boost-n-Buff for leads. I'll use the head with a 4 12 cab or a single 12 Bogner cube...depending on the gig size.
I've been using my Alchemist one of several ways, all of which sound great. I usually keep the green channel clean, and I'll either keep the red channel clean or slightly overdriven. I have been using three Barber pedals for a while now with great results--into many different amps, I might add. The signal goes Guitar > Tone Pump > Direct Drive > LTD SR > Yamaha GEQ > mods, delays, etc. Between the three Barbers and the GEQ, I can get different degrees and flavors of OD, from a compressed TS-type tone, to an open, Tweed-ish thing, to a Marshally grind. Sounds FANTASTIC with the Alchemist.
BTW, I have never owned an amp with so many killer, usable sounds inside. Every time I fire it up, I feel like I discover a new, interesting tone. Great, great amp--and I've owned my share of $2500+ boutique pieces.
Mesa V1 Bottlerocket & a Monte Allums mod Boss MT-2 on my 112 combo Alchemist. hits those meatier tones that the internal settings dont cover as well. great internal tones from clean to 80's-ish distortion. the V1 & MT-2 get me into that 'drowning in distortion shoegaze on the edge of metal' tone i like.